Not sure, you probably can. Ask again in 9 days lol
I'm pretty sure that you can't.
Given that they are Vampire hunters and whatnot.
Actually, I'd have a genuine problem with Bethesda if a Vampire could join. Mostly because it would show a massive lack of common sense.
I remember in Oblivion, there was a group of vampire hunters you could join as a vampire. Though they were small time, and it was a minor quest in the game anyway. --- If you are interested in an ES Roleplay story go to the board in my quote
Just like, how are you allowed to join the stormcloaks as a high elf?
Or as anything but a Nord, really, since they were kind of like Skyrim's KKK and hated everything that wasn't a Nord. But seriously, just because they Dawnguard are Hunters doesn't mean that they're very good ones. Sure, it would seem that way since they're getting all this hype, but look at the Blades. They were supposed to be the best of the best, and they got owned by the Aldmeri Dominion, which any Dovahkiin worth its helmet can slap around like toys. Besides, TES Vampires are very sneaky in a way that most regular people can't notice them, anyway.
Personally, I will be joining the vampires for that kick-ass Vampire Lord mode. Something about a spell that's pretty much a Darth Vader Force Choke and morphing in and out of that kick-ass form at will is pretty sweet. I guess technically, it's just holding a guy in the air with telekinesis and draining his health, but the power to hover and the bat-swarm thing is pretty cool, too. Only reason I might join the Dawnguard is for the crossbow, and I'm pretty sure you can just kill them and take it. --- You can't kick ass and have both feet on the ground at the same time.