One of the most interesting theories about the essential nature of the vampire comes to use from Lloyd Worley at the University of Northern Colorado. Worley identifies the vampire as one of the stock mythical figures of the our culture. Something that is changeable on the surface but derived from an enduring and fundamental fear.
Worley proposes that the the vampire somehow springs from our feelings about fetuses and babies. The fetus, after all, is sustained directly by its mother's blood for none months. While young babies are active throughout the night and demand nourishment directly from their mother's body.
Worley L (1991). The Prenatal and Natal Foundations of the Vampire Myth. International Conference for the Fantastic in the Arts.