They can be found at Oxenfurt.
They are weak against Moon Dust, Devil's Puffball, Vampire Oil, Yrden & Igni.
Contrary to popular belief, monsters are not all alike. Like people, individual members of the same species can each have their own unique traits, preferences and weaknesses. A good example of this is the katakan which once fed on the inhabitants of Oxenfurt. Perhaps influenced by its close proximity to the hard-drinking student youth of the city, this vampire had developed a keen appetite for blood spiked with a hefty dose of hard alcohol - and also baubles and gaudy jewelry of all kinds.
Vampires are incredibly creepy in The Witcher 3, mostly because they're more beast than man. These monsters can prove to be a challenge if you're not paying attention, but with the right tactics, you'll soon overcome their fearsome reputation.
Vampires are big, but they're not slow. They have a lunging attack and they can swipe quickly with claws. Almost all of their offensives will stagger you, so it's always best to avoid contact entirely until after they've finished their own attacks. Igni is key here, as vampires burn well, and can be staggered quite easily with a blast at the right time.
Their invisibility can be a pain, though. Usually between attacks, vampires will cloak themselves. You'll still be able to see a sort of shimmering outline of the beasts, but your attacks won't lock on to your target - although you can still do damage if you're lucky enough to swing and hit. Things can get tense as you watch a vampire prowl under the guise of its invisibility cloak, but as always, patience is key - wait for the creature to emerge before rolling away, and then strike back.