Board Thread:Lore Discussion/@comment-2165692-20160701070121/@comment-27516771-20160813200132

That's because of the dark magic the Vampirism was caused. Molag Bal is father of the Vampires and he's Daedric Lord, dark magic. Aetherius's magic is light and good and that's why the vampires are affected by it, it's the exact opposite of the magic that created Vampires.