I'm sure there will be a ritual of some sort if you decide to side with the vampires, where they give it to you wilfully.
Exactly and if you won't see it if you're a vampire already.
Not to mention that regular vampires are said to be getting a big overhaul in the DLC... So becoming one now may not reflect what it's going to be like playing a vamp with Dawnguard.
Who knows, maybe you'll decide the Dawnguard themselves are 100 times more awesome than vamps when you play the dlc.
imho, if you are not already a vampire, just wait.