You really expected an armor made of leather and iron (Dawnguard armors) and (going off the shoulder plates) elven/gold armor shoulderpadded'd leather (royal Vampire armor) to be better than dragonbone and Daedric?
In my opinion, you expected too much.
Castle Volkihar becomes your player home by the way, Kung if you side with them.
Its YOUR fault if you side with the Dawnguard, you get a tiny room on the top floor as such.
Players are spoiled these days and expect way to much of things, expect things that aren't promised.
You can return to the soul cairn by the way, you just have to walk out of the side door by the entrance of the soul cairn so you discover "Castle Volkihar balcony"