Does Dawnguard require Vampirism and if so is it permanent?
Hey! it does require Vampirism, and it doesn't. You can play the whole expansion fighting for the Dawnguard and never become a vampire. If you are a werewolf and become a vampire then you lose being a werewolf. I presume you can go back to the Companions and get re-wolfed, but I've not explored it. The thing is though, they introduce two new perk trees, one for werewolf and one for vampires, and the only way to get the perk points to fill t is to eat people as a werewolf and vampire. Of course there is an achievement for doing each of these so you may want to do them if you're a trophy hunter.
I played the game fighting the vampires as a member of the Dawnguard so what I did before choosing sides was turn into a werewolf and go on a massive kill spree the likes of which has never been seen. solitude will never be the same. It took about an hour or so to kill the 120 odd people necessary to max out the wolf perk tree. I saved the game after the carnage, a work mate mentioned to me after the fact that I could of done all that, popped the trophy and then just reloaded my game like nothing happened. Yup, Solitude is looking pretty barren of people at the moment.
Anyway, when you start playing you join the Dawnguard and pretty soon after you face the vampires. You'll be given the option at that point to become a vampire. If you choose to do so you will have ditched the Dawnguard and officially joined the vamps. So if you want to not be a vampire then say NO! Later on in the quest you will have the option to be a vampire again, even though you are a signed on member of the Dawnguard. At that point I chose to be one, did what I needed to do and then went on another mass kill spree to max out the vampire perk tree. When you return to the Dawnguard as a vampire they will shun you and send you off to Markarth (I think) to be cured. All you need is a Black Soul Gem to do it.
As far as I can tell that cure is supposed to be permanent but I've had to do it a couple of times since, the vampire thing seemed to come back. I don't know if that's a bug or if I got re-infected when fighting vampires.