I think that the Wii U will sell between 2 and 3 times as many units as the PS-Vita but it is difficult to say which without knowing about price cuts, game announcements, and the release of new hardware ...
If Sony could reduce the price of the PS-Vita to $200 with a bundled memory card and game, and they can get some big game announcements at E3, the sales of the PS-Vita could pick up to a level where it would make it unlikely that any system will sell 3 times as many units. At the same time, if that doesn't happen and Nintendo gets some big game announcements they could easily sell more than 3 times as many units as the struggling PS-Vita.