OK, I'd appreciate it if you guys could answer this question with as little extraneous information as possible, since I'm still a ways off from actually starting Dawnguard and I normally dislike even minor spoilers, but this question is bugging me; will my character have to become a Vampire in order to back them over the Dawnguard? My current playthrough is an evil Necromage, and I'm wondering if I can even play the DG story without either being a "good guy", or turning into a Vampire.

As little information as possible? I'll restrain myself, I promise.

To that question: Yes.
To your wondering: That dilemma is indeed forced on you.

If you want more than that, just ask.