Xbox One still has to launch in many countries which will probably cut down that number by several hundred thousand.  It still has Titanfall, Halo 5, Quantum Break, Mojang's next game, and other AAA exclusives to come out which will help push the system.  There's also the pricecut that will likely happen in the next two years, possibly as much as $100.  And there's the possibility of a cheaper kinectless SKU that would push even more systems.

Compare that to Vita which looks to have third party support in the West drying completely up.  It will likely depend almost solely on niche Japanese games and indies to push new sales.  It could see a pricecut but that could put it into the position of selling at a loss which Sony has always been reluctant to do.  It's appeal almost solely to very hardcore gamers likely means that a larger percentage the systems potential buyers probably already has one compared to a system with broader appeal. 

There's also the fact that gaming handhelds face far more competition and it only appears to be growing compared to gaming consoles.  This is likely due to the costs and risks of AAA gaming development keeping competition against gaming consoles in check.  Compare that to Xbox One which even though it faces very stiff competition against PS4 at least has a very weakened Nintendo console to help pick up the slack.

Bet accepted.