Don't cite sources on myths, DF doesn't follow the rules of any particular setting or universe
As a matter of fact, it does. Weaponry and technology is meant to go until
our XIV century. Fantasy in DF usually follows western fantasy, and it follows our mythology, since vampires is a myth definition of our world and we use it in DF, it is comparable. Not to mention I cited sources on the thing that
defines vampires, blood sucking. You can't pretend DF doesn't follow rules of any particular setting or universe, because it does, and it borrows
heavily from our world and our myths.
and a single book is hardly definitive of the immense variety of medieval folklore. Toady's not trying to do a strict recreation of Romanian folklore or anything, he's trying to create a fantasy world generator, influenced not just by mythology but by fantasy conventions and tropes.
This statement contradicts with your previous statement. It doesn't follow rules of any particular setting or universe but it borrows from our fantasy conventions and tropes? Nevermind that, as I said before, I was defining vampires with two books from reputable sources. I can't really put in here every source of folklore attempted and I'm certain if vampires here where like the ones from Twilight we sure would get people complaining here that "vampires aren't like that" because, as almost all other life, vampires are defined by certain parameters.
Vampires will eventually have variable traits, semi-random. Judging by the common depections of vampires in fantasy, effects of blood starvation could vary between each world/curse lineage:
-Become less human in appearance
-Become weaker
-Become stronger but more desperate
-Nothing, except they'll drain the first living person they get their hands on
-Revert to corpse
-Crumble
-Revert to human (though this one is rare)
-and more
A vampire can't turn back into a human. It's dead, by it's definition. It can turn into a corpse or crumble. But as random as they can get, if something doesn't get it's
sustenance for some time, it will
perish. Unless what is sustaining it is magic, like zombies, though if you cut the source of the magic, the zombie would probably crumble, or could crumble.
Like I said, it's not going to be constrained by the strict definition of any particular convention or setting, the only constant thing will eventually be consuming blood/life force. You can have the tragic vampire driven to murder by their hunger and survival instinct, or the evil vampire who doesn't need blood to survive but loves nothing better than to drink it.
It will be constrained, yes, by some constants. As you agreed before
Vampires are defined for being bloodsuckers, and that's about it. Lore varies wildly.
Not only being a bloodsucker, but being an undead. Vampires are, thoroughly the lore, undead bloodsuckers. If it's alive and it sucks blood, it's not a vampire. And if its dead and it doesn't
need to suck blood, it is not a vampire. So no,
you can't have a vampire who doesn't need blood to survive. It wouldn't be one because it escapes the definition of vampire.
In resume, while you can argue that DF doesn't strictly follow the rules of our world and that it alters our mythology, it use it's characters, like vampires. And it defines them, by our definition, the need to suck blood and it being undead. It can have powers as random as you like, but it needs blood for sustenance, and if something doesn't get what sustains it... If a vampire doesn't need to suck blood it isn't a vampire. Vampires in DF
need to suck blood, only the adverse effects of they not doing it are not implement
yet. Toady recognized it.
EDIT:
To close the case, DF itself say that a vampire is a creature "cursed to prowl the night in search of blood." If the part of not getting the blood is ignored, sure a consequence, a dire one, should happen.