Seriously? "we don't know what this DLC will allow you to do." seriously? You have to cure Lycanthropy to be a vampire.
I don't think Umezawa is asking if you can be a vampire and werewolf at the same time.
I think he's saying he wants to play a werewolf and be an ally of the Volkihar.
To answer your question, Umezawa, it's not yet clear from the information we have at this point. Maybe you can help the vampires without becoming one yourself. (Or maybe they will try to drink your blood

)