I agree with both answers I see below, but I think there is a more specific answer, as well. As a student of Carl Jung's depth psychology, I believe each generation creates, and is fascinated by, typal images from the unconscious that represent the struggles of that generation.
Vampires may represent emotional vampires, needy people who take our life and attention to meet their own needs. Zombies may represent people who are not mentally and emotionally alive, people who are not vitally connected to the joy and wonder of life.
For people in their early teens (and maybe for another ten years), it can be a shock to realize that our parents don't have it together. If your parents watch a lot of TV, perhaps have addiction issues of their own, created a broken family with you in it because they didn't know how to love and relate, it makes a lot of sense to look at how to live among vampires (apparently powerful people who prey on you for energy) and zombies (apparently living people who are actually dead) to find out what we all want: how to be a genuine, wonderful, alive person in this world.