It could be possible using tools like UMDGEN, and whatever tools were used for the PC versions, assuming they are compatible with the version you are trying to mod (which seems to be the case). You will need to know the fie structure of the game and check what to replace or mod.
The thing is, will the Vita be able to run higher-res textures even for a PSP game on Adrenaline? PPSSPP is better than Adrenaline in that aspect, but I'm not sure if the Vita could handle it in the first place, unless the textures are only slightly better and/or aren't that heavy.