It might sound paradoxical, but the reason that vampires have so much power over us is because we know they're not real.
The myth of the vampire was debunked as exactly that - a myth - back in the 18th century, but that hasn't stopped scores of writers, directors, game makers, and a whole host of other artists rendering them in paint, prose and pixels. In Issue 293 of The Escapist, Adam Gauntlett argues that the reason that vampires have endured for such a long time in fiction is because that every knows they're just make-believe, making it much easier for people to mold them into whatever shape they want.
There's no such thing as vampires.
That comes straight from the top. The question was decided a long time ago by the Church of Rome. Archbishop Davanzati's 1744 Dissertazione sopra I Vampiri was accepted by scholars both religious and secular, and it said Vampires were make-believe, nothing more ... That, I suspect, is how they've survived, even into the modern day. People's fantasies are often more important to them than their reality.
Consider how vampires evolved. To [Dom Augustin] Calmet, Davanzati, and their contemporaries, vampirism was wrapped up in religion. Calmet saw the devil at work; Davanzati saw a diabolically-inspired threat to resurrection of the flesh. If a modern horror author followed that line they'd be laughed at. This is a secular age, far more so even than the 18th century. Religion, though not dead by any means, doesn't have the hold over our imagination that it once did. To the modern era, vampirism is a virus, or a phenomenon of parapsychology.
Dracula gives way to I Am Legend; both are vampire tales, but with very different premises. Even the things we think we know about vampires - stakes, garlic, sunlight - are as much creations of the film industry as folklore, and owe little or nothing to religion. Meanwhile the holy symbol, if it works at all, is symbolic not of one faith but of faith in general, on the premise that it doesn't matter what you believe so long as you believe in something ... Vampires survive in our mythology precisely because they are creatures of fantasy, which allows them to keep up with the times. As our list of fears grows and changes, so too do they.
Gauntlett thinks that while stories about vampires are more commonplace than ever, if you strip the vampire down to just its core elements, it's clear that the real inheritor of the vampire's legacy is someone you might not expect. To read more about the evolution of the vampire, read Gauntlett's article, "Contemporary Immortality."