I personally think us humans have very vivid imaginations to conjure up such a beast, they do say however there is no smoke without fire, take the medieval knights of old who returned from far off campaigns talking of giant lizards (Dragons) that consumed man & horse, years later it was realised that these beasts did exist in the form of crocodiles etc, there is in fact a bat that is a blood sucker,
http://www.thewildones.org/Animals/vampire.html
So you see there is such a creature, as far as human form though I suspect there are & have been many cases of individuals who have claimed to be such creatures & followed the occult perceptions of such a being as far as they possibly could, there are I suppose many cases of mentally disturbed individuals too who have been afflicted by vampirism so I guess we can’t just dismiss their existence out of hand, the same could be said of the devil & god & there seems to be a lot of people who believe they exist!! :-/
I like to keep on open mind myself on such things, there are some very colourful people in the world, no doubt some who claim to be the real deal, maybe they are??? :-/