I do not know if anyone has found this but I did it on the 360. I first was a vampire and I asked Serana to turn me into a vampire. When she started to bite me I pushed be and mashed the power button with beast from equipped. It said vampire lord was added but I turned into a werewolf. at the end of the transformation I lost the beast form ability and had the vampire lord abilities. However I still looked human and was still accepted by the dawnguard. I only wish I could keep my werewolf abilities as well

Dude there is a Dawnguard hints and spoilers section.

That is interesting!

However there doesn't look like you can feed nor does there appear to be a cure since their the option with falion doesnt appear. I noticed vampire sight was missing as well