Here's the thing. In this last months, the Vits has been able to compete with the 3DS in Japan (solid 3rd party support and the PS4 helps). WiiU is more solid in general worldwide, but the latest japanese push for the Vita has helped the handheld more.
Also, unrelated thing. Vita has Minecraft and e WiiU doesn't. Is Microsoft afraid of a Nintendo system with Minecraft to compete for the "first console for my kid"' market?