PSVita supports multiple PSN Accounts per system, but only if you have multiple memory cards.

[Zlejedi]

smbu2000 said:
So would you still be able to share games then? My gf and I both have psp's, which are both activated on my ps3, and it's easy to share our digital psp/ps1 games between us. If content is locked to one account on a memory card for Vita it doesn't seem like it.

If you don't care about trophies you can always use someone else account or even create a new one for each memory card and put content on it to share it like normal discs.