So, there's something I'm slightly confused about regarding vampirism in lore. As we know, vampires are weak to the sun, which is a giant hole leading to Aetherius. However, vampires are often adept spellcasters, and have no inherent weakness to magic (just fire). And yet Aetherius is the source of magic. We know Daedra aren't weak to the sun, so is it due to their status as undead?
For example, Dawnguard has sun spells that do extra damage to the undead. Is this a reference to undead having a weakness to the sun in general, and that sun damage to the undead in general just isn't implemented in the game, but is in line with lore?

I've been confused on this for some time. Any ideas?

I believe it is the intense magic from the sun which harms them. The diluted amount that mages use is no where close to the same level of energy. It should also be noted that there were vampires in Daggerfall that inherently resisted fire instead of being weak to it.