My own way to introduce vampires:
Vampires have been around for a long time, as have many other types of monsters. They are all the Children of Cthon, lets say: creations and mutations resulting from the dark magic emanated by an evil god.
Several centuries back, however, some plucky mystic hero types managed to successfully banish and seal away Cthon, via ( lets call it ) the Montesi Formula. The flow of dark magic into our world was largely cut off, and this greatly weakened the monstrous forces of the world, causing them to die off, go into hibernation, or lose power.
In the case of vampires, this resulted in the species largely being wiped out over the ensuing years, as even those who survived and remained active were weakened, and made easy targets for vampire hunters. The result is that, by the twentieth century, vampires are so rare that even organizations with access to secret knowledge ( like SHIELD ) are likely to write them off as a myth, or some kind of one-off oddity. There's maybe one or two strongholds ( like an isolated place in the Balkans ) where a significant number still reside, and there's maybe a double handful of individual vampires otherwise loose in the whole world, mostly keeping a very low profile. Similarly, there's only a small handful of remaining vampire hunters, and they generally aren't hunting "vampires" so much as "this single particular vampire." One of those is Blade, pursuing the vampire who destroyed his life, Deacon Frost.
Of course, this all changes when, by chance or malign intent, the Montesi Formula collapses, and Cthon is free to start flooding the world with dark magic again, causing monsters to arise, others to gain might, and a lot of easy ritual magic to work again for those foolish enough to pursue it. . .
