Maybe the sun damage thing has to do with the fact that they are weak to fire?
Meh, I'm going to take a lore license and go with the Magnus/Aetherius thing. It's definitely more suiting to the TES fantasy setting than the light wavelength thing. I will fix my vampire mod so that you don't take sun damage in the SI.
Actually, the sun is just a giant hole in Oblivion, literally. And I would not fix your vampire mod, because even the daedric lords require the energy leaking fromk the aetherius to keep themselves as their own realm.
And there is a critical thing people are missing, vampirism is a daedric curse created by Molag Bal (the rapist, defiler, and corrupter) and/or the vaemirna (the prince of nightmares). Going with the nightmares part, they're being of nightmares. They can only really prowl the night, they're horrific looking, and are pretty much what nightmares are made of. Going with Molag Bal, they're an abomination to Arkay, they feast on the blood of the living, they were created because Molag Bal defiled a person horrifically, and so on. It's not that they're unholy, they're a daedric cursed group of mortals.
And lastly, don't only look at the OB brand of vampires. There's three different types in MW, a good amount of lore in the book Immortal Blood in OB, and there are a heck of a lot of different types in DF.
IIRC Vampires bodies arn't whole like man or mer, they are little bits of flesh and bone tied together by magica, so it could be that the type of magica that is holding the skin together could be highly suspetable to what ever force comes from the sun....then again I might be drawing from some other fiction, since I havent looked into tes vampirisism in a while
That is...so very wrong about vampires. What you gave was a way to prepare a skeleton for necromatic spells.