So in dawnguard you can become a vampire lord even if you're werewolf. Now the process would remove the power of the wolf in you, but it can return once you give up being a vampire lord. Now this process certainly gets my attention because we are removing our self from Hircine realm yet some how we can return once we get rid of the new strain of vamprism, so this begs to wonder what kind of power is being used to over ride the wolf blood in you and replace it with vampire lord blood?

Magic
Plain and simple magic

The power of Molag Bal friend!

Yes I know but it begs the question that a Daderic prince can over ride another prince magic so easily.

I think this pretty much sums it up.

Other than that I really can't imagine the reason for it. My caffeine-starved brain thinks this phenomenon might be somehow related to the Dovahkiin pledging her/his soul to several different Daedric Princes during the game, without facing major consequences for doing so. But I can't quite figure out the connection yet. (or my point...)

Well during the companions quest lycanthropy overrides vampirism so i guess its just what strain of the disease is stronger than the other. Like the companions version of lycanthropy has the power to override vampirism and the volkihar strain of vampirism has the power to override lycanthropy.