Vampires. Hm. Well..I'll give my two gil on this matter. The first major question would be 'is it possible to create something akin to a vampire in game?'. My reply to that would be very likely yes. Aether exists in the body and spirit, and it's likely blood can be used as a method to transfer Aether form one source to another as a catalyst. So as has been stated, someone becoming Voidsent-possessed or somehow enhanced could create an individual who gains increased physical or magical ability, and in turn has to consume blood (drawing Aether from it) in a method much like a vampire would. However, traditional vampire weaknesses such as sunlight and running water likely wouldn't do anything to the person so afflicted at all as it's more like a demonic boon than an actual vampiric curse. So it would more be an Aether-eater who uses blood as the transfer method than what we consider a traditional vampire by our own real world legends.
The major difference is there's one other thing that most vampires are able to do that such a creature wouldn't be able to. And that's create other vampires by the usual method. Voidsent curses, from what we've seen, are tailored to the individual (Amandine for example). While she attracted other voidsent with her presence, there's no indication she turned anyone into one in the mansion in Haukke. This means that a vampire as exampled above, in order to make new vampires would likely have to convince an individual to make a similar contract to the one they made, thus making vampires in this game more or less a type of voidsent cult rather than undead. And actually, if you think about it, with what we've seen with the Lambs, a vampire-inspired voidsent cult might actually make a fun story arc or something. But as far as our more traditional Nosferatu, it's unlikely it'd be the same.
The major difference is there's one other thing that most vampires are able to do that such a creature wouldn't be able to. And that's create other vampires by the usual method. Voidsent curses, from what we've seen, are tailored to the individual (Amandine for example). While she attracted other voidsent with her presence, there's no indication she turned anyone into one in the mansion in Haukke. This means that a vampire as exampled above, in order to make new vampires would likely have to convince an individual to make a similar contract to the one they made, thus making vampires in this game more or less a type of voidsent cult rather than undead. And actually, if you think about it, with what we've seen with the Lambs, a vampire-inspired voidsent cult might actually make a fun story arc or something. But as far as our more traditional Nosferatu, it's unlikely it'd be the same.