Referring to hardcore fans of Stephanie Meyer's "Twilight" series in which most of the main characters are vampires. The series has an especially strong following of curiously devoted fans. "Fanpires" can also be loosely used for people (usually pale teenagers clad in black) who are very partial to vampire stories/are greatly intrigued by vampires/think vamps are sexy.
Person 1: Geez, why don't they go out in friggin' daylight for once.
Person 2: They're fanpires, they don't do daylight.