Vampires have long been a multi-use subject for plots and metaphors in stories, and RPGs are no different. Vampires were present in the earliest rules for the most basic release of D&D as a type of powerful undead and were fleshed out further in the first Greyhawk supplement, and have been a staple of the game ever since. Almost as long-lived are the rules for playing them. The story goes that Arneson had people playing good guys and bad guys, with one of the players opting to play a vampire named Sir Fang, modeled off Christopher Lee's character in the Hammer Horror films, which prompted another player to base a character based on Van Helsing as played by Peter Cushing in the same films. As the character was fleshed out, they added in healing and curing abilities and basically created the first cleric class (called a priest at the time). There's also the tale of later games where a character could become a vampire by advancing through the undead ranks from a skeleton with some interesting results.
Editions come and go and by the time 3e comes around Vampires have developed a pretty large laundry list of abilities amassed from the vampire stories of the time: drinking blood, draining life force, incredible strength and durability, supernatural agility, mind control, shapeshifting, the ability to scale walls, mastery over creatures of the night, the creation of lesser vampire minions. Even better the stats were presented in the form of a template that could be applied to any sort of humanoid (such as a human or elf) or monstrous humanoid (such as a minotaur or medusa) so you could create a wide variety of vampires with various racial and class bases to fit the needs of your game. Well, that was the theory at any rate. In practice it's a little more difficult because you have to figure out what exactly such a template is worth. Slapping a bunch of abilities onto a character would make them far stronger than they were before, so how do you measure that?
The solution WotC came up with is the concept of Level Adjustment, where every template (and playable monster race) had modifier that indicated that the abilities offered by your template boosted your power to the point where you counted as a character one or more levels higher than you actually were. A race such as the drow (dark elves) possesses improved stats, minor magical abilities and spell resistance compared to their surface-dwelliing kin and thus have a level adjustment of +2., so with all the powers that a vampire has, you're looking a level adjustment of +8, so a character with 2 levels in a class and the vampire template counts as a level 10 character. This is a bit of a problem, because your vampire will have only 2d12 HP total because undead do not have Constitution scores on account of not having working bodies, and thus they don't add anything to their HP totals, so you're looking at less than 20 HP on your 10th level character (1st hit die is maximized). If anything gets past your defenses you will die to an at-level sneeze. Admittedly when you do you can probably float off and recuperate in your coffin, but it's still going to take you out of the action for a day.
If you were level 10 before a vampire sunk its teeth into you then you'd count as an 18th level character and you and your 10d12 HP (70.5 average) would be eligible for end-game content. If this happened during play as part of a plot event you'd be in a weird place because by the rules you'd just been slingshotted into the highest level bracket way above your friends, but yet still have the gear of a 10th level character and basically woefully unprepared for the challenges you're supposed to face without the infusion of half a million gp worth of magical items. Of course, you'd also be woefully unprepared for the challenges you're supposed to face just by the game's math- a monster of a given Challenge Rating (CR) is supposed to be something that can be fought by a party of four characters of an equivalent level and cause them to expend about a quarter of their daily resources (hp, spells, etc). A basic humanoid of a particular level is supposed to have a CR equal to its level, but while the vampire template adds +8 to your effective character level, it only adds +2 the CR because the game doesn't consider the vampire's abilities that much of a game-changer on the field of battle. The CR system is a pile of broken crap to begin with, but even it thinks that there's something screwy with playing a vampire in 3e.
While level adjustment pretends you're a character of a higher level, it runs into several problems with the level system itself. The most basic problem is that levels in any one thing come at the opportunity cost of not being able to take levels in another thing while characters advance in ability as they level- this means that any time you level outside of your main class you're effectively trading high-level abilities for low-level ones. Most of the time this is a horrible deal except in the cases where either your high level abilities are nothing worth writing home about, the low-level abilities are just that good, or you have no illusions of the game ever actually reaching the highest levels of play.
Compounding matters is that while you can take a level in any class, not all levels are created equal. A 20th level fighter is an improvement over a 1st level fighter- over twenty times tougher due to all the extra HP, plus better skill at combat through things like an improved attack bonus, extra attacks per round and feats. But a 20th level wizard doesn't just have over twenty times the spell count of a 1st level wizard, but a host of abilities a 1st level wizard can't even begin to comprehend, abilities such as enhancing your senses, traveling up or through walls, flight, remote viewing, shapeshifting, mind control, reanimating an army, predicting the future, teleportation, demiplane creation, absolute immunity to spells, mass homicide and time manipulation. Your old spells get better range, duration and damage, and you get better spells that have even more powerful effects, while your warrior friends merely improve on their base capabilities for the most part. This general phenomenon is referred to as "Linear Warriors, Quadratic Wizards" and means that for the most part that while a fighter could justify taking a level in barbarian or something because there aren't too many high-level fighter-exclusive abilities, it's rarely worth it for a caster to take levels in a noncaster class because you're sacrificing high-end spells for low-end features.
It gets more interesting when you factor in prestige classes (often abbreviated as PrCs), which were introduced in 3e as sort of an alternate advancement representing certain specialized options (of which there were hundreds). While multiclassing would normally hurt your caster character because you'd be sacrificing spell progression, one of the key features in many 3.5e PrCs was the spell advancement feature where they stacked with your casting class to determine your spellcasting ability (for example: the archmage). When you look at sorcerers and wizards you'll notice that wizards don't gain any higher level features aside from spells and bonus feats, while sorcerers have no high level features at all aside from spells. Thus there's basically no reason not to level into a caster-boosting PrC as fast as possible because you get to keep all your spellcasting features and get more features on top of that, basically turning you into caster+1. In fact, there was no real reason not to plan out your character build so that you could hit various PrCs for a few levels of features and then roll onto the next one in a form of character progression somewhere between a slalom and a pub crawl. It may have required monumental rules awareness, reduced the class system to an overly complicated talent tree and created character class summaries longer than this sentence, but it was the road to ultimate power in 3e.
When compared to all the power you could amass through levels, sacrificing eight of them for the vampire's abilities was unthinkable for a caster and not really worth for a noncaster either. Even the lich as the peak of the casting undead was an unfavorable trade with a level adjustment of +4; enough to lock you out of your highest level of spells in exchange for some immunities, stat boosts and a few special abilities (that could all be replicated by your spells). It got to the point where the lich template was a free party favor for achieving the 20th level of the Dread Necromancer class and the class was still just the middle of the pack of full casters when compared to the power of a wizard hopped up on PrC. The Unearthed Arcana supplement introduced rules for buying down your level adjustment by spending experience points so you'd only be a little bit behind your adventuring buddies and better able to fold your abilities into your character's advancement as they ceased to be worth the same amount of levels when compared to the capabilities of a higher-level character. Unfortunately, only a Level Adjustment of +3 or lower could be bought down more than once (and bought off entirely), and at +8 LA there are simply no rules for the reduction of the vampire's LA. Even if you decided to go through with this unfair deal, your +8 LA vampire is still stuck with a host of vampire weaknesses such as sunlight, running water and holy symbols; there's the Vampire Lord template that loses these weaknesses and gains even more abilities, but that's a template only for NPC monsters.
Pathfinder unceremoniously dumpstered the Level Adjustment rules so the question of "how do I play a vampire?" went back to "ask your DM." Exactly how much this increase in power should cost your character remains uncertain, but the DM is free to make shit up, presumably while also slapping various benefits on on other player characters to compensate. Back in 2012, Paizo released Blood of the Night, a short (~32 pages) player companion book about various types of vampires, including rules for playing as dhampir, a race of half-breeds descended from the undead (and often dedicating themselves to hunting down and/or brooding about their undead kin). While it provided no concrete rules for determining a vampire character's value, it does include various vampire character options, such as feats, spells, and rules for hunger and withdrawal.
Hunger rules are almost as key to vampires as rules for all their sweet powers. Even if the game doesn't immediately come with them, it will probably show up sooner or later. Back in 2e, the Van Richten's Guide to Vampires supplement for the Ravenloft setting introduced its own rules for vampires, and gave them age categories that let them advance in power and ability over the years (not unlike dragons).
It also introduced feeding rules: A newly-created fledgling vampire needs to drink 12 hit points worth of blood within a 24 hour period, consuming 1d4 HP per round of drinking blood (or 12 HP per round if the vampire rips open the victim's throat and drinks that). A vampire can't drain a target past -10 HP since the target is dead of blood loss at that point, and should the victim survive, the damage can be restored through spells or natural healing as normal. As a vampire ages, the amount of blood it requires per 24 hours is reduced by 1 point per age category, so the top of the line patriarchs only require 6 HP per day (but generally really want to drink more than that). Should a vampire go 24 hours without consuming enough blood it will lose 1 Hit Die (and all the bonuses that come with it) and effectively loses one age category per missed feeding when it comes to determining its strength, magic resistance and sunlight tolerance. These can't be reduced below the Fledgling level, but it can lose HD until it hits 1 HD, at which point the vampire goes feral and attacks any source of blood it can find. Each day of successful feeding will restore the vampire by 1 HD and 1 age category, so it's best to keep up on your drinking habit.
Incidentally, here's the rules for what happens if a PC is turned into a vampire:
Van Richten's Guide to Vampires posted:
Vampires With Surviving "Goodness"
It's entirely up to the DM if a particular newly-formed vampire retains some part of his or her mortal attitudes, emotions, and beliefs upon the transition to undeath. For DMs who like concrete rules, try the following:
If a character is killed by a vampire, and the creating vampire is destroyed or leaves the area before the victim rises as a vampire, roll 8d6 and compare the result to the victim's Wis. If the result is equal to or greater than the victim's Wis, the newly formed vampire is completely and utterly Chaotic Evil. If the dice roll is less than the victim's Wis, however, there's a possibility that the new Fledgling vampire might retain some portion of its previous world view, possibly including alignment. (See Chapter XII, "The Mind of the Vampire.")
IMPORTANT NOTE: This does not mean that a PC who becomes a vampire can remain a PC! The only purpose of this "rule" is to give DMs the opportunity to add some role-playing spice to vampires. To repeat, a PC who becomes a vampire immediately becomes an NPC, under the complete control of the DM.
The rules are: screw you
3e doesn't use Ravenloft's age categories for vampires and the ensuing loss of power from abstaining, but it does have rules for general undead hunger introduced in Libris Mortis, a book whose rules for undead cover raising them, controlling them, fighting them and even playing as them.
The most horrifying thing about these the level adjustments
Various undead may dine upon various different things such as blood, flesh, bone, or energy. An undead creature's diet is further broken into one of three categories: Not Required, Diet Dependent or Inescapable Craving. If it's in the Not Required category, the food is just something the undead creature enjoys, but suffers no penalty from missing meals. Should a creature go too long without a meal, it must make a Will save every so often or have its Wisdom score reduced by the gnawing hunger, which will further reduce its Will save bonus. An undead reduced to 0 Wisdom is a ravenous creature who will pursue its chosen food even to the point of self-destruction (and if it's a player character it's under the DM's control until it feeds and recovers its Wisdom). For a Diet-Dependent creature, it's a DC 15 Will save every 3 days to avoid 2d4 Wis damage, while a creature with an Inescapable Craving needs to make a DC 25 save every day without feeding or take 1d6 Wis damage. Wisdom damage recovers at the rate of 1 per day only if the undead creature is feeding regularly, so going too long without a meal will put you in an incredibly bad position unless you have a fantastic Will save (but even then you'll need to watch out for rolling a natural 1). Unfortunately for the Vampire, they're Diet-Dependent on blood, but also have an Inescapable Craving for Life Force (acquired through their level-draining energy drain attack), making them (and their spawn) the only undead with two diet necessities to manage and thus two ways to be driven into a hunger-induced downward spiral. Even more reasons not to play one in 3e.
When compared to the hunger rules in Van Richten's Guide and Libris Mortis, Pathfinder's (optional) rules look more like a hybrid approach. As in Libris Mortis you need to make a Will save each day after going without food for a number of days equal to your Hit Dice, but unlike that book the DC is equal to 10 + 1/2 your hit dice + 1 per previous check, so the difficulty will scale as you level and the longer you abstain. Even if you have a fantastic Will save and don't roll any natural 1s to autofail the check, the DC will eventually exceed your abilities after a month or so, so you'll fail and go into Withdrawal. Withdrawal is, as the name suggests, not good, because it provides a penalty to all your abilities that scales the more Will saves against hunger that you fail. The most obvious penalty is to Will saves themselves, ensuring that you'll spiral down even faster against the rising DCs. Your Strength and Charisma scores suffer an increasing penalty, so not only will you be weaker, but as an undead creature your Charisma governs your HP and the DCs of your special abilities. Other abilities that are penalized include your ability to disguise your increasingly gaunt frame in order to pass among the living, as well as things like Channel Resistance, Damage Resistance and Fast Healing if you have any of those abilities that make it harder for people to destroy you. Should the penalty to your Strength or Charisma scores equal or exceed your ability scores you go completely catatonic and can only be revived from your helpless state if someone else feeds you. On the bright side, you only need a single meal to remove all the penalties and reset the grace period to a number of days equal to your Hit Dice.
If you're thinking "withdrawal penalties suck but at least the game isn't taking control of my character away from me," you'd be wrong. An undead creature in withdrawal needs to make a Will save at the same DC (and with the same Will penalties from withdrawal) every time it's within 10 feet of a helpless source of food, with a failure sending the creature into a feeding frenzy where it can do nothing else but attempt to feed until it's had a meal. Unlike with some other sources of fear or compulsions, a successful save doesn't immunize you for 24 hours, so you're still on the hook for as long as potential food is nearby. So not only can you not be trusted to stand watch while your allies are asleep, but if any of your allies are knocked out or paralyzed while in battle, you have not-terrible odds of making a bad situation worse (especially considering that you're already going to be underperforming in battle while under withdrawal penalties). The hunger rules aren't like regular hunger, and things that would protect normal creatures from hunger (such as a ring of sustenance to remove your need to eat) are explicitly called out as not working against undead hunger because it's not really hunger. In the game's words, "the act of feeding can be likened to that of an addict satiating her inner demon." Which is interesting because this set of rules is more mechanically punishing than an actual drug addiction.
The way drugs in Pathfinder is not unlike the way they worked back in 3e when they were introduced in The Book of Vile Darkness (along with rules for things like demon worship, torture, S&M, and human sacrifice). Basically when you take a hit of something like opium, catnip, or some fantasy drug such as shiver you gain a temporary benefit for some amount of time plus some amount of ability score damage to represent its toll on the body and then must make a Fortitude save to avoid addiction. The rules for addiction are a little different depending on the book.
In the BoVD, you need to fail saves for the initial primary and secondary effects of the drugs to affect you, and addiction works according to chart of different strengths ranging from Negligible to Vicious. Higher ratings have higher Fortitude DCs to avoid addiction, and shorter periods between satiation before you have to start making Fortitude saves to avoid ability damage (with higher rating drugs doing more damage to your system). Unfortunately, every two months you spend addicted to a drug with a rating other than negligible increases the addiction rating by one step, so 8 months of using simple low-threat drugs will mess you up as badly as even the worst stuff out there. Succeeding on two successive saves to fight off the ability damage from withdrawal means that you've kicked the habit, but if you take it up again you'll need to make saves against the same DCs as before. Spells such as lesser restoration can deal with ability score damage while remove disease cures the addiction, while greater restoration or heal do both.
In Pathfinder, both the primary and secondary effects are automatic and addiction is a disease in the most literal game mechanic sense of the word. While addicted to drugs, you take a certain penalty to your ability scores that persist whenever you're not under the effects of the drug, but since most drugs only affect you for a couple minutes to a couple of hours, that's going to be most of the day. If you take a dose of a drug while you still haven't recovered from the ability damage of the previous dose, the DC to avoid addiction goes up by 2 per dose. This is a bit of a problem because if you're under the effects of a Moderate or Severe addiction you cannot naturally heal the ability damage caused by the drug you're addicted to, which means that without magical restoration it's just going to pile up in your system until your ability score hits 0 and you are rendered paralyzed, catatonic or just dead. If you're addicted to elven absinthe then you're going to take 1d4 Con damage that will not naturally heal, and drinking another dose means you're just taking 1d4 more Con damage and increasing the DC to recover by 2. If you're addicted and don't have access to things that remove ability damage then five drinks during the addiction period will kill an average dwarf (or an above average human or elf) through 5d4 unrecoverable Con damage.
The upside to this is that addiction is actually rather easy to recover from. As a disease, you need to successfully make a series of two or three consecutive daily Fortitude saves against the disease to kick the habit, and the DC decreases by -2 for every day you spend not using it (down to a minimum of the base addiction DC). But unlike other diseases, the penalty from addiction is just a persistent one instead of a cumulative one, and there's no penalty for failing the saves aside from the fact that it's just going to take longer before you succeed several times in a row. You may feel crummy the entire time, but you won't get worse if you take a hit and there isn't a single mechanical element that will mentally compel you to do so. The only way you're taking more drugs is if you decide your character is taking more drugs (just like in real life, right)?
If you do have magical access, not only can you use spells to remove ability damage so you can keep doing drugs without dying, but you can use spells to kick the habit entirely. If you go a day without using drugs then someone can use remove disease to break its hold over you on a successful cast (which isn't even the most unexpected thing remove disease can get rid of- that award goes to being impregnated). And if you're actually immune to disease then you can't get addicted at all, meaning that one of the best classes for handling substance abuse and/or casual hook-ups is the paladin thanks to its Divine Health feature (plus the ability to use your lay on hands feature to cast remove disease to help your fellow adventurers in vice). Doesn't matter what you're doing, you can always quit when you want to. Despite being the leader in vice-related shenanigans, the antipaladin isn't immune to disease, just the negative effects and thus could be addicted but not suffer any downsides because of it. Maybe it's just a little voice in the back of your mind telling you to do more drugs, but can you ever be cured of your non-addiction after doing enough drugs?
But if immunity to disease makes you immune to addiction, does immunity to poison make you immune to the effects of the drugs? It's hard to tell. On one hand, drugs are specifically part of the poison descriptor for spells, the drug rules mimic the poison rules in their descriptors and many real-world drugs probably qualify as some form of toxin, but they specifically go out of their way not to refer to drugs as poisons, meaning that they can probably bypass poison immunity. Sadly, undead and constructs are immune to all effects that require a Fortitude save anyways (unless it's harmless or affects objects) so they're immune to addiction, probably immune to the initial effects and certainly immune to ability damage, so no toking with the spirits (unless they're psychopomps).
But if immunity to disease blocks addiction and immunity to poison blocks drugs, then the best class for doing a mountain of cocaine with no ill effects whatsoever is the monk thanks to having immunity to both disease (purity of body) and poison (diamond body). There's even the precedent of the drunken master archetype, which takes the fighting style whose movements involve mimicking those of a drunk and adds real alcohol to the mix (which the game counts as an addictive drug if frequently used). One minor problem: archetypes involve trading some class features for others, and among the features the drunken master archetype replaces with its own features are the monk's immunity to diseases and poisons. Your substance abuse is going to tear your party and liver apart. Meanwhile the alchemist class gains immunity to poison while the monk-like internal alchemist archetype also picks up immunity to disease and as an alchemist you will probably never be short on interesting concoctions to shove up your nose. If you want super powers, an unnatural body and an affordable substance problem, be an alchemist and freebase a dragon. It's certainly better than being a vampire.