i used to frequent a forum dedicated to vampires and the consensus of opinion was that not only were there hybrids but that sanguinarians can learn to feed on energy when no blood is available.
My personal belief is that what we are actually taking in is the life force which flows through the blood, which is why stale blood is much less satisfying than that which has come straight from the body. The same energy is taken in through psy feeding.
I also believe that the nature of a vampire is a non human soul inside a human body which is why we have to take in additional energy to sustain us. We simply can't take in enough through the normal human means.
Further, I believe that the term 'vampire' covers a number of beings which have different natures, and they all derive from the same source, the nephylim. Nephylim souls have passed down the ages to where the only vessel available was the human body and so here we are.
These are my beliefs. I am firm in them but I accept I could be wrong