Every writers depiction of Vampires are different. In Harry Potter we don't see much of them but they do exist and aren't much of a problem. So what are they. Well, I have a good theory, I think that Vampires in Harry Potter Canon are a parasite that takes control of dead bodies and uses them as a puppet. This would explain why they have a longer lifespan but are not immortal. Also this would make them just another species rather then evil demons and satanists. Also this isn't too far from vampires in folklore because in folklore vampires were thought of as soulless corpses coming to life to feed on the blood of the living. Usually demons possessing corpses. Also these vampires are vulnerable to Garlic which Garlic fights death and decay. Plus Garlic is used to fight parasites.
If Vampires were anything like their depiction in The Vampire Chronicles, The Vampire Diaries, Twilight or Underworld then dark magic wizards would choose to become them. In Harry Potter Canon they are called evil creatures. If they we're humans forcibly turned like Werewolves they wouldn't be called evil. Also they are living dead and there is no mention of being revived and mutated as a consequence in the entire franchise. The only way to come back in Harry Potter is either leaving an imprint to become a ghost or using Horcrux's to be reborn with a part of soul staying on earth. So Vampires being a parasite or demon using corpses is more likely and there are no demons in Harry Potter so a parasite is likely. Also let's a assume that Zombies in Harry Potter are also a parasite just feral and stupid making them nothing like Vampires. And Inferi are just corpses reanimated by magic similar to zombies in voodoo folklore.
The only evidence against this theory is Victor Ketsueki, however, he was probably lying about becoming a vampire to get attention. Also the Vampires in Harry Potter canon have names relating to blood like Trocar or Sanguini. Most humans don't have names like that so them being a separate species makes more sense. Also we know that human-vampire hybrids, known as Dhampir in folklore and modern literature exist in Harry Potter. Perhaps if the Parasite keeps the bodies functions working then they can cross-bred with humans. Also I doubt these vampires burn in sunlight, I bet they're just Nocturnal like in Folklore, Bram Stoker's Dracula and Carmilla. Also Sanguini is described as having dark shadows under his eyes and being emaciated. I think if a parasite grows inside corpses and keeps them working then they would meet this description.
Who knows maybe will get an official answer to our questions in the future. Perhaps a future Fantastic Beasts movie will have a vampire character. Perhaps will get a Pottermore post about them. Perhaps will never know. Well until then let's just assume that Vampires in the Wizard World are a species of parasites that reanimate human corpses and live inside them.
