The Vita has an advantage in the fact that Sony does not seem intent on releasing another handheld. So, they're going to leave the thing on the market for as long as it will sell, without a direct successor to steal its (limited) momentum.
But the Wii U has always outperformed the Vita on holidays by a decent margin of at least a few hundred thousand, and it has a few games left in the tank that will provide a substantial boost (Mario Maker, Zelda).
If the Wii U has two holiday seasons to work with, I think it could be up by about a million by the end of 2017, and I'm not sure the Vita will close the gap. However, I am fairly certain the NX will be here by the end of 2016, so I'll sadly have to say the Vita will probably outsell the Wii U... which is kind of sad, especially since I feel the Wii U had a much better catalog.







