I wasn't going to do this, but eh. Infodump time! xd
Vampirism.
Porphyria at best, self-delusion of people who just want to be something more than human at worst. Psi-vampires are just sensitive to the feelings of others around them, and sanguines do not have the extended lives as they claim. They just like the taste of blood. Trust me, I dated one once. He was in his late teens and looked 40. And I know he was in his late teens as his aunt produced a birth certificate and various dated baby and toddler photos at one point in the relationship. She loved stripping his dark and evil image on him. 3nodding
In the case of those who are actually porphyric, the lack of heem in their systems makes them crave blood. Usually manifested in a love for rare-cooked meats, but it can make the more impressionable believe they're real live vampires and send them after blood from human sources. Also, in all but two types of the disease, it can make their skin rupture and blister badly in sunlight, which is one of the reasons porphyria is often associated with vampirism historically. Other reasons include sensitivity to certain foods (most notably garlic), obsessive behaviours (which may explain all those old "vampires need to count grains thrown at their feet" myths) and if left untreated, barking madness, which explains aversion to holy water, crosses, running water and the like.