These creatures of darkness will never lose their appeal because they are us. We see ourselves in vamps. They represent the human struggle in many ways, at other times they symbolize the uninhibited animals we sometimes wish we could be.
Take, for example, the "good" vampire. Whether it's Edward from the Twilight Saga, Stefan from the Vampire Diaries, or Bill from True Blood, or Louis de Pointe du Lac of Interview with a Vampire, these characters struggle with who they are. They despise themselves and their nature as dealers of death. They try to overcome their darkness and live for good. Tell me that is not the most human struggle of all. Tell me that is not the true meaning of humanity. We fight our inner demons every day, fighting off darkness, trying to be upright and good. Hasn't there been a day when you were tempted to do something you knew was wrong, but you wanted it so bad? Did you have the self-control to resist? Often when a vampire is hungry or angry, they are depicted as having actual physical changes to look more demon-like. What if our own ugly natures manifested themselves so? My god, perhaps we might hate ourselves too.
In opposition to this "good" vampire is the "bad" vampire. Think Eric of True Blood, Damon of Vampire Diaries, or Buffy the Vampire Slayer's Spike. This character knows his ugliness, and accepts it, embraces it, even displays it for all to see. He is beautiful to us because he does not hide who he is. He is most often the character who will be sexualized, because the audience can live vicariously through him. There is a dark side to everyone, but we deny it to ourselves. This character doesn't. He breaks all the rules we are constrained to obey. We choose to conform to the fabric of society, but there's a part of us that admires the person who doesn't, or doesn't have to.
This analysis could be expanded in a hundred ways: we like the promise of undying love (pun intended), absolute trust, sexualizing restraint, glorifying self-control, hoping for redemption, respecting life, wielding power. It's all of our human nature rolled up and put into a creature who may exemplify the human condition better than the human can himself. You can complain all you want that vampires are stupid and your girlfriend likes Twilight too much- even I admit there's should be a limit to an obsession. But don't pretend you don't find yourself strangely attracted in some secret corner of your heart to the vampire, because he is you, and isn't it human to love yourself?
