ask yourself if MH5 came to vita would it sell more than 3mil? There is your answer

Seriously though as much as it'll do well on vita (a million seller imo) its fanbase is heavily on 3DS right now. 3.6mil+ counting

Edit: Why did you put the West in your OP?

No.

It's like you're saying Monster Hunter isn't popular enough in Japan to make people buy a console for just that game.  Oh wait.  The 3DS sales had a massive boost in Japan each time a Monster Hunter game released on it.  From that, I'd say if one was released on the Vita, an exclusive, not yet another port, I'd say the same would happen.  The Vita would have massive week on week sales.

I'm just saying.  The 3DS weekly sales boost after their release just proves that it doesn't matter if the fanbase is on one console or another.  The game will sell systems.

*edit*

Also pointing out sales that Monster Hunter 3 on PSP is much higher than Monster Hunter 3 on the 3DS.  Monster Hunter Freedom Unite on PSP is much higher than Monster Hunter 4.  Going by sales It's possible that there are also some fans of Monster Hunter that won't purchase the 3DS versions.  So there's a small possibility that it'd sell better on the Vita.

No, only if MH5 would be released on Vita first and 2 or 3 years later on 3DS.
Simultaneous release: 3DS version would sell much more. Many MH fans already have a 3DS.

No it won't.  Monster Hunter Frontier not only requires a monthly fee, but it's supposedly the worst in the series.  It's also on the Xbox 360 and PC right now.  When it's ported to the Vita it'll also be on the PS3 and the Wii U.  

So a game that requires a monthly fee, is a port that's on five systems, compared to an exclusive that requires no monthly fee.  I'd say the Vita is being set up for failure.

If people want to keep good support from Capcom for the Vita they have to buy the game, because otherwise it is clear that Capcom has no market on the system.

Not as a late port, no. Would Monster Hunter have sold more if it were a DS exclusive? I don't think so, and that's saying something. People in Japan buy systems for MH, simple as that. It doesn't matter what platform it's on.

Monster Hunter Freedom 3 isn't Monster Hunter 3. MH3 3DS is a late and enhanced port of the Wii version.

Monster Hunter 4 sold more units in it's first year in Japan (3,23M in 3,5 months) than Monster Hunter Freedom Unite (2,77M in 9 months).

MH4 is already the third best-selling MH ever. It hasn't released in the West and isn't that long on the market by now. It is on it's way to outsell MHFU on PSP in Japan (sold more units in comparable time-frames) while lagging behind MHF3 in the same league. So it is between the most successful titles in the series if you compare launch aligned sales. It hasn't released yet worldwide. So it has a shot to outsell MHFU (the most successful MH overall), if it comes to the west. Basically this all says, that the platforms doesn't matter much, it would have done the basically same on Vita.

http://www.vgchartz.com/tools/game_comparison.php?reg=Japan&ending=Cumulative&game%5B%5D=Monster+Hunter+Freedom+Unite+%28PSP%29+%5B18542%5D&game%5B%5D=Monster+Hunter+Freedom+3+%28PSP%29+%5B44133%5D&game%5B%5D=Monster+Hunter+4+%283DS%29+%5B63447%5D&game%5B%5D=