Vampire

Vampires are a type of monster in myths and folklore (ref here). They are also a type of mutant (ref here), introduced in Fallout 3.

Real Vampires
Vampires exist across the Capital Wasteland, and are a type of mutant.(ref here) They are often mistaken for cannibals (ref here), but possess numerous abilities humans lack, including sharpened teeth (ref here), enhanced strength and agility (ref here), varying degrees of psychic ability (ref here), and naturally, the ability to live off of blood, extracting it from human bodies in a way not possessed by humans, that leaves the victim alive afterwards (ref here). Unlike fictional vampires however, they cannot turn into bats, and are reflected by mirrors. (ref here)

The largest group of vampires in the Capital Wasteland is The Family, a gang led by Vance for at least 6 years (ref). It's members have come from multiple other locations, including Rivet City, Megaton, and Paradise Falls.(ref) Some have been attracted to their home in Meresti Metro station, drawn by the presence of other vampires. Usually however, Vance has come out to them and seeked them out with his ability to sense other vampires. Vance has used the idea of the fictional vampire to lay down rules to guide the Family away from cannibalism, and instead have them act as a civilized group. (ref)