I can't believe there are so many people that seem to think there is even a slight chance that Vita could sell anything even remotely close to what Wii U will do. It's really baffling because WiiU sure will have much much much more big system sellers than Vita among other huge advantages in comparison.
If I had to bet I would say:
Vita : 15-20 Mn
WiiU: 80 - 100 Mn
Really two totally different situations.