It's been awhile since I've shared how vampires work in my story universe, and I've made some changes to things here and there since I first came up with this stuff. So, since I've been drawing some vampires lately, I figured I'd share the updated version! Also including the info on werebeasts for completeness, even though I haven't changed much there. I'll probably write up some info on other things such as fairies, demons, ghosts, etc. at some point, when I finish deciding everything for them.

Anyway, here you go!

VAMPIRES
In this world, vampires are a lurking menace, dangerous and feared by those who know they exist. They do attempt to keep their existence secret, so that people will be unprepared for them, but they don't always succeed -- some especially brave people, called Hunters, even train themselves to take out vampires, though this is of course a very dangerous occupation. Although vampires initially retain the same personality they had in life, their lifestyle tends to bring out the evil in people, as have very strong hunting instincts and cannot help but see the living as food. A good person won't automatically become evil, but this is detrimental to them, so they'll likely eventually adapt. Similarly, people who were evil to begin with tend to become the most successful vampires.
Creation: In order to be turned, a person must be nearly or completely drained of blood, whilst also consuming a bit of a vampire's blood before they die. They will then remain dead for about a day before awakening as a vampire, typically mad with hunger until they've fed for the first time. It's customary for a newly-turned vampire's sire to stay with them awhile to help them learn about their new 'life' and get the hang of things, even if they decide to part ways afterwards. Just being drained of blood isn't enough by itself to turn somebody, so most vampire attacks don't result in more vampires. Since vampires become sterile upon being turned and can no longer reproduce, 'half-vampires' are impossible.
Undeath: Vampires are undead. By all clinical standards, they should be dead -- no vital signs, etc -- but some form of unholy magic keeps their soul with their body and causes them to stay animate. They no longer age and cannot be killed by most conventional means. As a strange effect of this condition, they completely lack a reflection, though they do still cast a shadow. Although they no longer need to breathe, some vampires still do, due to force of habit. They have no body heat of their own, so their body temperature is the same as their surrounding environment, like a corpse.
Physical Changes: Vampires look very much like living people, with a few exceptions. Though their skin isn't always visible beneath their fur, it will take on a paler tone. They will always have large, sharp fangs, which is more conspicuous on some species than others, and their claws will usually be slightly larger and sharper as well. Their irises also glow, though this is only properly visible in certain lighting. In many cases, vampires are not easily identified on sight, which is advantageous to them.
Diet: Despite being clinically dead, vampires require nourishment to fuel their existence, and they get hungry just like living people. They of course get their energy from blood, with fresh blood from living people being much more nutritious and desirable. The blood they drink is absorbed via their stomach before going to their veins, where it's slowly used up over time (thus a well-fed vampire will bleed more when cut). If they already have enough blood in their veins, any extra can go towards gaining fat and/or muscle instead. Conversely, a prolonged lack of blood can cause them to lose weight, though they can't actually die of starvation...a critically starved vampire will instead simply lose all control and mindlessly attack the nearest person. They seem to be retain the same metabolism they had in life, thus some vampires require more blood than others. Although they're capable of eating normal food if they feel like it, it's nutritionally useless to them. They're affected by (and can even become addicted to) various substances such as drugs and alcohol just as living people are, but they're immune to being poisoned and cannot overdose.
Powers: All of their senses are greatly enhanced, they gain amazing night vision, and they are even able to hone in on the sound of a heartbeat from quite a distance away. A newly-turned vampire is just slightly stronger than they were when they were alive, proportional to their muscle mass. Their physical strength increases with age, as well as how many vampires they have created. They usually also have enhanced speed and reflexes (though to what extent varies from one vampire to the next) as well as being able to move more quietly than logic dictates they should. They use either fresh blood or reserves of blood to completely heal any injuries (with the unusual exception of the bite marks from when they were turned, which remain forever), and they heal faster the more well-fed they are; they can't regenerate lost body parts, but they can still heal good as new if said body parts are reattached. As vampires get older, they often will eventually start to manifest one or more special powers, such as thrall, voluntary shapeshifting, flying without wings, mind-reading, etc. These abilities generally start off very weak and slowly become more powerful with time and use.
Weaknesses: The most foolproof ways to kill a vampire are a stake through the heart, decapitation, or exposure to direct sunlight. They also burn more easily than a living person, so fire can be another effective way to kill them. Silver, holy symbols (crosses, etc.), and holy water all greatly harm them, but they can eventually regenerate. Bright light can temporarily blind them or distract them with pain, as their eyes are extremely sensitive. Garlic can sometimes work as a vampire deterrent, as it tends to irritate their nose and mess with their sense of smell (though some vampires are more sensitive to it than others), but it doesn't otherwise harm them. They also cannot enter someone's home unless they are invited in by someone that lives there. When a vampire is killed, their corpse turns to dust.

WEREBEASTS
Werebeasts (usually referred to as were<species>, as in 'werewolf', etc.) are considered a much lesser threat than vampires, since they're only dangerous for one week out of each month and are always safe during the daytime. Although some are quite successful at coping with their condition and not harming people, some simply cannot control their rage and are dangerous due to often having the desire to hunt and eat people while transformed.
Origin: The original werebeast was a werewolf. No one knows precisely how old they are, but it's speculated that the condition arose as a spontaneous magical mutation. It's a dominant trait and is passed down to anyone with werebeast in their lineage. Wolves and dogs are most commonly seen with this condition, though with the increasing open-mindedness concerning inter-species relationships, some other unrelated species may have this condition due to having a werewolf ancestor.
Transformation: Regardless of species, all werebeasts have two forms, their normal form and their 'wereform'. The wereform is essentially an enormous quadrupedal version of their regular self with elongated fangs and claws, even on species that don't normally have these attributes. In this form, they become much stronger but much more primal in intelligence -- a semi-sentient predator. They take on this form at night during the seven nights around a full moon (the full moon and the three days before and after it). They're strongest and have the least self control on the night of the full moon, and this is when they're the most dangerous. The transformation is involuntary. A werebeast who doesn't want to harm people will often spend that week somewhere remote where they can hunt non-sapient animals instead, since that satisfies their instincts just as well.
Powers and Weaknesses: Despite having some supernatural/magical qualities, werebeasts are still mortal. They age normally and eventually die of old age, though their lifespan is longer than that of non-werebeasts. They have a 'sixth sense' for other supernatural beings (such as ghosts, vampires, other werebeasts, etc.) and can also smell magic. They're immune to being made into vampires, and their blood is both unappealing to vampires and harmful to any vampire who tries to drink it anyway.
In Normal Form: They have a heightened sense of smell and hearing compared to normal. They can be killed in all of the same ways as anyone else, though they're more resilient when recovering from injuries and may sometimes survive serious wounds a normal person would not. They also have a taste for meat, even if they aren't a carnivorous species.
In Wereform: They regenerate amazingly fast. It's extremely difficult to kill them. The exception to this rule is silver -- wounds caused by silver weapons will heal at normal speed, thus they can indeed be killed by a silver bullet as the popular legend states. They also cannot regenerate lost limbs, so decapitation is another (albeit more gruesome) way to kill them. Their taste for meat also expands to include even sentient people (excluding undead -- they like their meat fresh), coupled with a very strong desire to hunt things. In this form, they also have extremely keen senses of smell and hearing and overwhelming physical strength, though their intelligence and inhibitions are greatly reduced, especially during the full moon transformation. This can make them easier to catch since they may not be smart enough to notice traps, but makes it difficult to communicate with them.
Werebeasts are indeed very good at fighting vampires when in wereform, if they choose to (they'd need to be well-fed first to be interested in hunting something they don't intend to eat, but they'd enjoy it). So that's another possibility for a 'good' werebeast. Whilst a super-old vampire with strong special powers (like Vlad or Desdemona) would likely be too much for them one-on-one, a werebeast is still extremely unlikely to actually get themselves killed in such an encounter, and the average vampire is at a disadvantage...especially since a vampire will likely try to avoid using their best weapon (the biting) against a werebeast, due to their bad-tasting and harmful-to-vampires blood.