I think a lot of different aspects of the vampire mythos vector into their appeal in general. As for their current popularity it seems that during troubled times socially and economically people turn to fantasy in general as an escape. Why vampires are the escape du jour is probably different for different people.

The Twilight and Vampire Diaries vampire appeals to teenage girls I think for obvious reasons: an older, protective (yet still really cute, oh squee) guy who was hard to get, seems a little dangerous but would never hurt you or even really be sexually threatening, symbolic or otherwise, owing to his fabulous self-restraint. ::cough::

I think the attractive, misunderstood (??), but possibly very dangerous outcast archetype appeals to more than just women, though women seem to flock to that vision of the vampire. Still, even guys like True Blood.

But maybe more important is how the vampire reflects us back to ourselves.
Blame it on Barnabas Collins for being the first anti-hero, sympathetic vampire perhaps, but Anne Rice and Francis Ford Coppola took us inside the vampire's black little heart and we found ourselves in there. A vampire exists in an eternal state of need, in pursuit for all time of what he has not. Whether it is the mortal longing for union with the Immortal or the Immortal hungering for the flesh and blood of the mortal, it is the myth of Psyche and Eros played out over and over. That ceaseless state of longing for the Other strikes deep in our archetypal minds.
Even the most inhuman and unsympathetic of our vampires, Stephen Kings vamps of 'Salems Lot f'rinstance, we can still see that desperate driving hunger for something outside the seeker.

Blood? Yes, of course. "The Blood is the Life", hot, earthbound  and mortal or the else the Elixir of Life and key to immortality, we want it.
So do our vampires and I think we like to watch them act out our hunger.