A Monster Hunter hitting Vita late next year wouldn't really change the Vita's fortunes, as two Monster Hunters would have already hit the 3DS by that point. Any Monster Hunter fan is either going to already own a 3DS, or will be planning to buy one since their friends have one. Monster Hunter is all about multiplayer, so buying a Vita for the series would be like buying a PS3 instead of a 360 for online multiplayer in the US. It doesn't make much sense when all your friends own a 360.
And I agree with Khuutra. There isn't really anything to convince a PSP owning Monster Hunter fan to buy a Vita aside from game line up, and their favorite franchise is heading to 3DS.
And I think that's a missed opportunity for Sony. I think that had they locked up Monster Hunter on Vita, it would've created a certain synonymy between Monster Hunter and future PlayStation portables, thanks to better implementation of the PSN. Friends lists, trophies, and downloadable content would tie the fanbase to their platform to a degree, which would helped in locking down the franchise for future generations.
Much like how Xbox Live penetration ensures that Call of Duty will be on the next Xbox as soon as it can be.