The connection between vampires and werewolves is an accident of history.
Way back in the early days of cinema – the 30’s and 40’s – Universal Studios made a several horror movies, such as Dracula and The Wolf Man. As time went on, Universal made more movies, some of them involving characters from previous ones. Sometimes characters from different series met, in an early example of a cinematic universe.
This early cinematic universe is the reason vampires and werewolves are linked in the popular imagination. However, looking at a list of movies that are considered to be a part of the franchise, I’m not seeing much particular interaction between werewolves and vampires in them. But there are other reasons for the two creatures to be linked.
An element that The Wolf Man introduced into the myth of the werewolf was transmission. The story of The Wolf Man is the story of a man that was bitten by a monster and realizing that he was turning into a monster himself, making it an early example of personal horror. This is similar to how vampires are said to reproduce, an element introduced in the novel by Bram Stoker.
Another thing that was in the novel was Dracula’s ability to turn into a wolf. This wasn’t something Stoker made up; vampires turning into wolves was an element in then contemporary Eastern European folklore, with the vrykolakas of Greece being a notable example. But the thing is, Dracula had a host of magical powers, and his literary descendants have followed suit. So much so, that some works postulate multiple species of vampire, each with their own unique power set, with only the theme of parasitism to unite them.
This parasitism is inherently horrific. Vampires have a need to feed on – and harm – other sentient beings. Even if a writer jumps through the hoops to sanitize vampires, such that they only need a mouthful of blood every few weeks, or even capable of living entirely off of animal blood, the fact that they drink blood from puncture wounds is still a bit creepy. It also gives them a reason to hide their existence from normal people, their natural victims, in much the same way a disease benefits from being asymptomatic.
Werewolves, meanwhile, are defined by their ability to turn into a wolf. Unlike parasitism, this is not naturally horrific. If a man can turn into a wolf at will, painlessly, and keep his mind while he is a wolf, most people would be comfortable calling him a werewolf, even though there’s nothing horrifying about his condition. This is the root difference between werewolves and vampires.
An effect of this difference is in trying to justify why each species would justify trying to hide their existences. As I stated, vampires naturally benefit from trying to hide themselves, in the same way a virus benefits from hiding themselves. Even in worlds where the supernatural is taken for granted, it’s still easy to believe that vampires would try to maintain a fiction that they shouldn’t be.
Werewolves, meanwhile, have the opposite incentive. While it’s understandable that the less dangerous kind of werewolf might go along with someone else’s masquerade, the kind that takes after The Wolf Man wouldn’t. They have every reason to tell people about their condition, so that people know to avoid them on the night of the full moon.
All that said, while vampires and werewolves are held together by historical contingency, I will continue to gleefully use that connection in my work.