I feel as though there is a considerable lack in clarity as to what ideas have actually been proposed for the Dres vampire council. So for my part:

I have actually been suggesting that the Dres are blood vampires, though they do not need to be for the concept to work. Here, to me, are the traits portrayed by such vampires in Morrowind and how they would play out for the Dres:

-indefinite lifespan: the surviving councillors are all over four thousand years old, but they are basically still mortal; they can be and some probably have been killed. This is the main reasons Dunmer see vampires as such an abomination, the Dres councillors included. They have permanently omitted themselves from ancestry, which to Dunmer is a big deal. A bad deal.

-harmed by sunlight: as wise women of the semi-nomadic to sedentary Saltlanders, the Dres councillors weren't really the outdoors type originally, and probably kept to the highest level of their strongholds where non-clansmer couldn't see them. This one is pretty minor all things considered.

-various buffs: these might give the wise women a leg up over your ordinary commoner, but while I think the Dres councillors should be powerful at what they do, they don't do combat. I don't think they should really provide a challenge to the player in a straight-up fight, certainly compared to Redoran and Telvanni of a comparable or even inferior rank. In short, any buffs other than the indefinite lifespan gained through vampirism are beside the point.

-thirst for blood: as far as I can tell, in Morrowind lore vampires have no insatiable craving for blood or anything, and drinking blood naturally doesn't make them look normal or anything like that. All it does is restore their health, and they seem to like the feeling of superiority over the cattle. So there are two ways I think this could go: either the Dres councillors just do without blood because they find sucking on beasts (Dunmer wouldn't even be considered) distasteful, or the vampirism they gained specifically gave them a craving for blood so that they would be reduced to sucking on beasts. The point being that they either don't suck blood or don't enjoy sucking blood.

-ability to spread vampirism through 'bites': they would elect not to do this. Being vampires themselves is an abomination; the last thing they'd want to do is spawn more abominations.

-clan association: this one's an interesting point that needs to be expanded on for vampires in general. For the Dres councillors, however, it's pretty inconsequential: all of the councillors gained vampirism through the same pact, receiving the same traits and effectively became a new clan, and refuse to expand their clan.

-clan traits: I'm not sure here, other than the possible dependence on blood mentioned above. The focus here is to give their 'clan' some quirks so that it's interesting and stands apart from other vampires, but by their nature they already stand apart. Either way, though, any positive effects should be negligible.

 

Overall, the point of the vampirism is that the council is corrupted, certainly in the eyes of the Dunmer, though the player may or may not agree. They are still, however, trying to steer Dres and the Dunmer in the right direction. If their secret is known, the Dres will turn against them, kill them, and in doing so will trigger the deal with the Sload. The councilors are not powerful, they don't meet up in nefarious councils and gloat on their evilness and inherent coolness; they're just wise women who keep to their quarters in the clansteads as wise women do and try to lead their tribe, except that they have an indefinite lifespan in return for selling their souls.

The possible and ideal bonus of them being vampires is that we can maybe actually make vampirism interesting and worthy of existing in the world of Morrowind, rather than just beng lame generic fantasy nonsense crammed in for the sake of it.