Just to reiterate:
The Vita only supports one profile per memory card, and that profile is linked to a PSN account. What that boils down to is one PSN account per memory card. Each game downloaded is directly associated with that profile, and the PSN account can not simply be changed by logging into another, as the system won't let you do that. The only way to switch PSN accounts is basically to reset the system back to factory settings, and of course that means you will no longer have access to the content that was on the memory card prior as it won't be there anymore. Even if you back the games up with something like CMA, it won't matter as those particular copies of the game will only verify and work with the particular PSN account they were bought with on a Vita actively associated with that PSN account.