Here is a WaPo piece which suggests it has to do with the transition from adolescence. I recall another piece suggesting it had to do with the female fascination with gay men (is there one?).
Vampires are hardly "my thing," but I do like early Anne Rice, The Night Stalker, Herzog's Nosferatu, and I thought Coppola's Dracula movie was better than its reviews. On the other hand, I couldn't get five pages into Twilight. (Should I try True Blood?)
I believe vampire books and movies offer a few attractions:
1. You know from the beginning that the plot twists will have to be extreme. Few movie makers offer up vampires who think pensively, talk inordinately, and live out ambiguous endings, sitting around in coffee shops. A real vampire story is going to deal with death.
2. We are fascinated with the idea that people may be something other than what they appear to be. You will notice that discovery and detection of vampires often plays a key role in the plot lines, sometimes commanding an inordinate amount of attention.
3. Vampire stories offer a platform for exploring the theme of pure, limitless, and eternal desire, yet without encountering the absurdities that might result from planting that theme in a realistic, real world setting, such as a man who loves cheese studded with raisins above all else.
4. Vampires play "hard to get" with women and they (for a while) embody Old World ideals of chivalry, in a plausible [sic] fashion. Yet since they are fundamentally different beings, we can enjoy watching their strategies while simultaneously distancing ourselves from them.
5. Men may like vampire movies for date movies, for uh…priming reasons. The movies prompt dramatic, emotional reactions in their companions. Women may feel that such movies "test" how their men respond to highly fraught stories, with a potential for demonstrating protectiveness. Or vice versa.
6. Vampires do not seem to mind social disapproval, and in this sense many teens look to them as role models.
7. Some of the popularity is arbitrary with respect to the vampire theme itself. There is a clustering of production in any successful cultural meme, once that meme gets underway. You might as well ask why there is so much heavy metal music today.
8. Viewers and readers, who know vampire lore and thus vampire vulnerabilities, feel better informed than the high-status people who, in the drama, are fighting the vampires.
9. There are few successful songs or paintings about vampires, so the story-based aspects of the topic appear to be important in setting their popularity.
Here is an unorthodox answer to the question.