This theory inspired from the Books of Enoch claims that vampires are the offspring of the union between the Watchers (Fallen Angels) and humans. 

When the Children of the Watchers had consumed all of the food available, they turned to mankind and began to eat their flesh and drink their blood. One of the legends says that a single Nephilim  survived by hiding in a cave that was sealed water tight. Hence, was  able to propagate a line later.

In another adjunct, vampires are the offspring of the daughters of Eve (female humans) and the Angel of Death sent by God on Earth. Vampires have the mission to control and thwart the demonic offspring of the fallen angels.  More about fallen angels.