If the vita gets a price cut and GTA next year (which I think it will), it should sell about 5m-5.5m. I expect the Wii U to sell just over 6m next year. So...Wii U will sell about 1.25x the Vita.
My prediction threads:
Wii U will sell under 40m units (made on 14th September 2012)
PS Vita will sell under 20m units (made on 30th September 2012)
Wii U will sell under 7m in 2013 - I was right