You lost me at Rayman Legends will be the best selling on the Wii U. So it's going to sell better on a console with a 3.5 million userbase instead of a combined userbase of the PS3 and the 360 of 150 million. It's not even going to be close. I would wager that the Vita version has a decent chance of outselling the Wii U version.
It can happen.
If you look at the latest Sonic racing game, it goes Xbox 360 290K > Wii U 250K > PS3 250K > Vita.
Despite the difference in userbase the difference between Xbox 360 and Wii U is just 40K. And for a game like Rayman - which was once exclusive - there`s a chance it can beat the HD twins. Especially if you remember that it`s coming next to GTA V.