Vita doesn't need to outsell the 3DS. All Sony has to do it stick to their guns, and rack out quality software. The initial lineup is tens of leagues better than what the 3DS had at launch.
I don't buy systems at launch....ever. However Vita has a flare to it, and to me that says something.
As for the 3DS. I'll buy one eventually, but the lack of quality software (besides the upcoming Resident Evil Revelations because that looks awesome), the lack of a second analog (no i'm not buying that over sized accessory), and the 3D are just downers for me.
Clarification on the software: Nintendo has great first party software for Nintendo fans, but when you aren't into the whole Mario thing you kind of get left out in the rain. I already own Zelda/MGS3 so I don't need another copy.