Good question and I wonder the same. Wii U is supported pretty heavily compared to Vita, yet Vita sells better. Hmmm.....
I have both and love both immensely. They provide very different experiences from each other and exclusives on both are amazing. I wouldn't call Vita a beast, but I also wouldn't say it's selling better solely because of Wii U's failure. I guess for Vita, maybe the Sony brand name power, handheld portability, and exclusives are broader-reaching?
I hope Nintendo's next console and Son'y next handheld (if they do one) sell better than their predecessors. It really is a shame when quality products sell so poorly.
It'll be awhile before I figure out how to do one of these. :P