Of course it figures that after I write my post, I come up with a few more ideas.
Vampires have a very powerful seductive ability, typically. I'd attribute this to pheremones. Normally, we don't notice them too much, so they don't play a very strong role. My guess is that these guys either have enhanced pheremones and/or pump them out like crazy. Maybe they can even control the release.
One of the trickier issues is the spread of vampirism. It would be easy to attribute it to a virus, but that doesn't end the problem. It's not a unheard of assumption for a virus to change the phenotype of a cell. It's the entire basis of gene therapy. However, usually viral infection is associated with problems in at least some tissue. For a virus to replicate, it has to destroy the cell it infects. So, for a virus to affect the entire organism, why doesn't the vampire die? It could be that the virus can replicate in some tissues and not in others, such as appendix vs. muscles, but I'm not certain such a mechanism is possible. Typically, if a virus can get in, it can multiply.
Hm . . . well, I've run out of new ideas. Anybody else have some thoughts?