However given the nature of Dwarf Fortress (and that's why I asked Toady about specifics to round up this debate), if Toady chooses to we could have vampires that simply suffer from blood thirst but really don't needed to survive, being basically thinking zombies that don't root and might have some extra powers.
Yup, it would be an undead with a blood lust. If Toady chooses it, wouldn't make sense, as it would escape the definition of the word vampire.
My position to have negative consequences on a vampire that don't feed upon whatever the Dice God decides (once that is implemented) goes beyond the folkloric fidelity and is more focused on gameplay balance. You should be able to yes, allow or seek out to have a vampire only fortress that yes, would have combat advantages but it should not come with it's own disadvantages or at least extra share of challenge.
I don't believe Toady or DF are worried with balance, to be honest. I think that's the last thing he cares too... I mean we have Minotaurs that a wardog can kill and we have creatures with syndromes so deadly and made of adamantine that it's just ridiculous. Also, it was never about folkloric fidelity or balance, but what defines a creature.
Such challenges could be anything really, given a procedurally generation process we could have vampires that don't need to drink blood but just to regenerate and heal injuries in one world (a interesting option and a compromise between your and Hugo position) and have vampires that will burst into flames if don't drink blood at least once every year. However all this is highly speculative, hence, again, why I asked Toady about specific possibilities.
Eh, they would still need something to give them sustaining. See, creatures in DF sustain themselves, or they should do it. We had that bug a long time ago that meant all kobolds didn't eat and they went extinct in world generation. Of course we have creatures like Bronze Colossi that are sustained by magic, but it's not the case of cursed creatures. A werebeast and a vampire are cursed, and the vampires curse is needing to feed on other's life force.
Now, I know people will give me the whole "it's Toady game he can make a vampire who doesn't need blood" and it is right, however, it would be wrongly be called so. You see, if we had a Bronze Colossi made out of Iron, and it was called Bronze Colossi in game, would it be rightly called so? Similarly, if we had an undead creature that drinks blood but it doesn't really need it for sustenance, would it be a vampire?
I know it seems I'm being stubborn, and sorry for insisting in this point, but using a word carries that definition with it, so if we call a creature a toad in game, we will expect it to the have the things that define a toad. If the creature is called a vampire or a rakshasa, there are different meaning and standards coming with it. It could die in the sun or not, it could transform in a bat or breath fire, but if it doesn't need to other's life force... If nothing, DFTalk #20 shows Toady is willing to commit to standards defined in myths:
Threetoe: Okay, so the next question is from Onebedterrin, he asks, 'Will there ever be any colossus other than bronze such as a stone, adamantine, or bone colossus.' He mentions we already have titans made out of different materials ... so yeah, the stone colossus, or the bronze colossus, what's the thought?
Toady: Well, the bronze colossus of course is a Jason and the Argonauts ... from Talos coming in and lifting up ships and shaking them and so on ... this is the reality ...
Threetoe: Yeah they've been there from the beginning ...
Toady: It's the reality we grew up with, all the Ray Harryhausen stuff. I think all of our megabeasts have appeared in a Ray Harryhausen movie somewhere, except the randomized megabeasts but every other one has been in either The Clash of the Titans or Jason and the Argonauts or one of the Sinbad movies. It's just how we roll here at Bay 12 Games. I don't think there's going to be therefore, another kind of colossus add as a raw stock creature and the randomized creatures that we currently have. You can have, as you said, a Titan made completely out of stone or something. They don't work very well right now, especially the ones made out of snow or whatever, they just die, but that would probably be how they come about. We already have the smaller magma man type things and so on and again, I think we're probably going to be moving in the direction where material creatures of different sizes are just things that randomly occur and hopefully it doesn't get too mushy or gobbledygooky but that's how it's going to happen.
Anyway, even if Toady answer to yours and my question saying "yes, they need to feed" vampire only fortress and undead accountants should still be possible, be it via modding or you would just need to feed your vampire regularly.