That would depend on what 3rd G ends up -being- and what it implies that 3G -was-. Now - i'm treading on glass here - so i'm going to make sure that this is to highlight that we _simply_ don't know. To further clarify, my guess would be that Capcom and Nintendo have a huge -NINTENDO- multiplatform strategy for MH right now and i don't expect any MH of note to be revealed for Vita any time soon. I agree with what you are saying and suspect we may well get MH3rdG PSP - but unless something drastic has happened, Vita may well not get announced unless Sony port it themselves (?)
ANYWAYS, that said - At the moment the (and my!) assumption is that the 3DS is the premium version. But what if that is not the case? Around the time of Capcom going to Nintendo 3rdP was just out - they could probably see the 3DS as a good fit for a port up of the Wii version, outsourced to 8ing. IF MH for Vita turns out to be a significantly upgrade version of MH then we're in an all new territory. However, after seeing MH4 presentation.... i doubt a scenario where investment to upgrade for any theoretical Vita version would be made! Alas for Sony, right now the only cast iron Vita machine mover at the current price tag for Japan would be a *premium* monster hunter game that's Vita exclusive (i.e. online from the Vita, graphical upgrade, megaton of new content).
Indeed it is! The 800k prediction isn't as controversial as people think, it was wrong but the bulk of the userbase is STILL up for grabs. 1.4m out of a 5m pie means MH can have a lot of sway (though for Sony that window will slam shut come MH4 3DS).
Certainly can see the dialemma - Sony need to push Vitas, but then again - PSP still has life in it. For Capcom, a PSP version seems to make a whole lot more sense right now. Again, it'll be fun to watch