Immortality. I think we find vampires as something interesting because it invokes our want for eternal life. Some people don't want to face the fact that they may one day, die. This is a scary thought, no doubt about that. Vampires don't die, unless stabbed in the heart with a stake. This idea helps us escape from our reality. It poses as therapy for us, and helps us cope with our daily lives. Sometimes, it also fills in for our denial from the truth. This also leads to eternal youth. Although it has only recently been that the vampires are young and good looking, it's the idea that they can be like that forever makes it all the more interesting.
Excitement. What is better than going to school, working, and socializing with our friends and family? I mean, our life? Of course, something that is not the norm. Our lives can be quite boring. How do we spike new excitement? We find interest in things that may or may not exist. Knowing that vampires may exist, and having being chance that they may not exist, makes it okay to be interested in it. The more suspense, the better.
Our interests. I don't think I know anyone who didn't grow up watching Disney movies. What do most of them have in common? Love. Recently, vampire movies have a lot of that. The idea of living "happily ever after" really means a lot when we think about vampires. Well, they are immortal. Sometimes there are forbidden love as well, or complicated love stories, which only enriches our interest in the stories.
Sex sells. Vampires weren't always portrayed as human like, young, and good looking. Back then, they were more ugly. They slept in coffins and had really white/pale faces, and sometimes seen as old. Now they're young, and beautiful. They sleep in beds, and have feelings. Why wouldn't it not be interesting? Vampire movies portrays our fantasy worlds, basically. And that portrayal works for many of us.