See - i realise this is semiflippant on myside, but the point is that the 20m or so Japanese PSP users haven't migrated anywhere yet and i suspect right now they aren't interested in the Vita -or- the 3DS. Hell, the PSP was the biggest selling machine here last year.
I do again reiterate though that there IS going to be a mass movement of MH fans for MH4. Absolultely. 100%. Guaranteed. That's the -real- "oh bugger" clock that Sony has to work against. If they've no MH of their own, then they need to beef up their line up.
3DS - 4,282,142
PSP - 2,079,152