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) Though mistaken for cannibals (ref here), they possess several characteristics that humans lack, including sharpened teeth (ref here), enhanced strength and agility (ref here), varying degrees of psychic ability (ref here) and the ability to live off of blood, extracting it from human bodies in a way that leaves the victim alive afterwards (ref here). Though they share their namesake with their fictional counterpart, Vampires lack the ability to transform into bats and are also reflected by mirrors. (ref here)

In the Capital Wasteland, a group formed by Vance named The Family reside inside Meresti Metro Station and have existed for at least six years(ref), with its members originating from various other settlements, such as Rivet City, Megaton and Paradise Falls.(ref) Others have been attracted to the metro station, drawn by the presence of other vampires. Due to Vance's precognition, he has the ability seek out and sense other vampires. The ideals of The Family were adapted from that of fictional vampires, in order to lay down rules to guide them away from cannibalism, and instead have them act as a civilized group. (ref)

Perks
The Night Person perk in Fallout 3, Fallout New Vegas, Fallout 4, Fallout 76, and Fallout: The Roleplaying Game, all depict vampires, and provide bonuses to the player only during the night. The Hematophage perk in Fallout 3 also depicts a vampire, and is given to the player by Vance if they befriend The Family, allowing them to heal from consuming blood packs. The Fallout 76 perk Bloodsucker depicts the act of vampirism, and allows the player to heal from blood packs and satisfy their thirst.