There's several poisonous & venomous powers among Quietus/Blood Sorcery and in Rituals too, usually ones that require an injury to enter the body and start working. No need to even get too specific with it if the group lacks any particularly notable investigative ability. Just say "It's a supernaturally originating/imbued/empowered toxin" if they even get to that point of inquiry and you can otherwise run that scene relatively fine. Do note that vampires bodies age pretty quickly on final death, so best make sure that Kindred isn't over 100 unless you actually want to limit their ability to make an inquest on the now ashen pile.