Vampires and other blood "drinkers" like witches are simply those that drug others and perform person to person blood transfusions.
The mythology comes from the imaginations of the victim's families and possibly hallucinations from the drugs or delirium from blood loss. It's literally that simple. You can become a vampire yourself if you have the proper blood type (AB+). Lineages of vamps and witches are those who passed down thier blood type and the knowledge of how to do this vile act.