You're talking about an undub. And yes, they are certainly possible. I'm not sure about this particular game as I have no experience with it.

It's generally not all that easy, there's not going to be sound files you can just pull out and replace with the Japanese versions sound files and expect it to work as such. Before you get started you should be aware that tri-Ace games tend to be very complicated, and convoluted (and although I have no definitive proof to point to) I'm under the asumption that VP's dialogue is done in such a way that the game will freeze (at specific points) if the sound gets out of sync in specific ways. If this is true then it would explain why so many people had issues with the actual game not working properly on the playstation. Of course it could also have been an emulator caused issue as well, honestly I don't know, just an assumption.

Although I suppose if there's clearly defined sound files, and they're exactly the same size, and the coding in both versions of the game is exactly the same your method seems theoreticly sound! Alas I have a feeling it will not be so.

You're going to have to find out how, and from where it pulls up the voices (if you don't have this information already)

Last question is of course, why undub? Of all the PS era games with voices, I thought VP's was the best (aside from Metal gear); this is personal opinion and you should neglect it =p

I know nothing about the PSP, and nothing about VP for it (if it uses the same voices? If it's a direct port etc.)

If the PSP uses the same architecture as the playstation, and the game's just a direct port than you should be able to use the exact same method that they used... Again I doubt it, but I can't say for sure, as I don't know.

First thing you should do is ask them, if the stars align it will make things easier, if they don't you're going to have to find out how, and from where the game pulls up the voices via tracing, if you're still interested in the project.