The Legend of Korra - Book 2: Spirits

 Spoiler

What actually happened to Vaatu?
One can't live without the other, right?
So where he at?

 Spoiler

Honestly, I feel like that's the biggest unanswered question of them all.

From how it was explained to us, in every other situation before the Avatar, Ravah and Vaatu fought until one was dead. But Vaatu wasn't technically killed, he was purified. There's a huge difference there. Purified could mean that Vaatu is gone forever, or at least for a longer amount of time than normal. It could also just mean nothing at all in the grand scheme, and in 10,000 years he'll be back.

Of course, that brings up the quote from Ravah where she says that when dealing with Light and Darkness, one cannot exist without the other.

I doubt this question will ever be answered unless the Season 3 and/or 4 plot is "New Badguys that plan to revive Vaatu like Jinora revived Ravah." Obviously they will be called The Order of the Black Lotus .