Just spent a few hours investigating this, if you are a vampire, and you become a werewolf you will no longer be a vampire, nor have any vampire abilities. Also if you obtain the vampire disease, and do the quests for the companions you will also lose the disease and not turn into a vampire. If you are a werewolf, you cannot become a vampire due to being immune to diseases. The only thing I have yet to try is to become a werewolf, and remove the immunity to diseases, but I know for a fact this will not work. When you become a werewolf, it will ALWAYS revert back to whatever race you were when you became a werewolf via companions quest, for some reason it will automatically revert being a vampire back into it's base race, no matter what, and technically, as far as game files go being a vampire is considered another race, there is a parallel vampire race to each race. It looks like the ONLY way for this to be possible is for some awesome person to make a mod for it.