There really are vampires, The Doctor’s met them. But the vampires of Venice are something else entirely: sea-dwelling refugees from the destroyed world of Saturnyne.
The true form of such creatures is a dark, leathery beast resembling a shark. They hide that body beneath a perception filter that manipulates the brainwaves of whoever’s looking at them. Sometimes, they can’t be seen in mirrors. In their natural form these vicious predators live underwater.
To continue their species the disguised survivors of Saturnyne drank from their female human victims until all the liquid had been removed from them. Then, they replaced that liquid with their own blood. Gradually, the alien blood changed the victim into one of the aliens, ready to mate with the male aliens, hiding and feeding in the Venetian canals.