This is actually a very pertinent question given our current fascination with vampires. The answer relates to a particularly Gothic theme, which is the way that Gothic elements disrupt supposedly fixed binary opposites. In the case of vampires and of zombies, the binary opposite that is disrupted is that of dead/alive. We feel we have a very certain and fixed idea of these two states, how we define them and how to identify them. The existence of vampires and zombies presents us with a terrifying possibility: that the boundary line between these two states might not be so firm and fixed as we had first suspected.
Vampires are living proof (if they did actually exist, that is) that you can somehow disrupt these two states and meld them together to create a form of walking dead being, who is dead in one sense, as their hearts have stopped beating, but terrifyingly alive in another. This fear therefore shocks us and taunts us with the possibility that our secure states of alive and dead are not actually as secure as we might think. It is perhaps this that gives vampires their position of perennial fascination, and it is this fear and fascination that is explored in this film and in other manifestations of vampire stories.