Board Thread:Lore Discussion/@comment-2165692-20160701070121/@comment-30677166-20161204175651

Vampirism was created by the Daedric Prince Molag Bal for one purpose; to spite Arkay (the Aedric god of life and death). The planes of Oblivion are likely comparable to pockets of existence within Aetherius. There are four known realms within Aetherius, Sovngarde being only one of them. Sovngarde is the afterlife of the Nords/Atmorans and it stands reasonable that while in this realm of Aetherius they are protected or isolated from the influence of Arkay. I believe Arkay is the primary reason for the suffering of Vampires in Mundus and since he is not the ruling deity of Sovngarde (Lorkhan/Shor is) it only makes sense vampires could exist here without being instantly obliterated.

As for the sun itself, it is simply the largest hole ripped to Aetherius by Magnus when he and the other et'Ada fleed from Mundus to Aetherius. This also explains why vampires are not inherently vulnerable to magicka because mackiga seeps into Mundus from Aetherius through the sun. At least a small portion of this magical energy is likely related to or comes from Arkay or contains his presense.