The PSP was only successful because of Monster Hunter, and now the Vita's main player is gone. Even before MH was announced for the 3DS, it was starting to gain steam and sales. I am not saying in any way the 3DS shouldn't get Monster Hunter, but I see no reason to not release it also for the Vita.

The Vita isn't just failing because of Monster Hunter is gone, but if MH came back it could help. 

Here are some reasons:

1. Built in analog stick, no more claw, it really would just be a perfect fit.
2. Stronger system, even if it was just a port of the 3DS Monster Hunter it would still look really nice on the Vita.

Some other things could also include those who bought the Vita because they believed there was going to be Monster Hunter on it. Hinted by the fact that Monster Hunter Portable 3rd was released on the Vita in Japan alone.

I do believe that if they did release it on the Vita, it would not only bring up sales, but even in the west it would be a success. Already with the fact that it's out on multiple platforms so there is no worrying about the lack of an audiance.

So please Capcom, release Monster Hunter on the Vita. Heck, we wouldn't even mind just a a english translated version of Portable 3rd to the Western Shores.