So-

I admit I'm kind of a metagamer and OCD. I did the dawnguard questline and had Serana turn me into a vampire before the soul cairne, because, hey, crossbow AND vampire lord? What's not to love!

But I realized I didn't do some of the side quests for the dawnguard, and when I returned, they wont talk to me. Now, that's fine, since they did give me a quest where I could get my vampirism reversed.

But if I do that, can I still ask Serana to turn me back into a vampire, or am I SoL?

You can ask Serana to turn you into a vampire whenever.

Are you sure? I thought only after the main quest is over.