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edit: misread, lol, just porting is a maybe, it really depends on the complexity of the game, as well, the vita uses shaders for it's graphical pipeline, and alot of cheap stuff doing in shaders is much harder to do on fixed pipeline, where you'll have to do more post processing, etc.
in short, it might be theoretically, but can be very difficult, and if the game is already maxing out the capability's of the vita, their's no way for it to run on the psp.
Just as feasible as porting a PS3 game to PSP. The Vita is powerful enough to fully emulate PSP games, so there's a huge gap. A port of any game wouldn't be possible without significant downgrades.
Oh, I read this backwards lol. But uh PSV to PSP? Probably not unless it was significantly downgraded. The other way around is possible though. Level 5 is making a game right now for PSP/PSV that the PSV version just looks like the PSP version with higher resolution.