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

[brendude13]

I think tying PSN accounts to Memory Cards is a great idea.

Everyone can have their own personal memory card that they can put into any system to log onto their PSN account and play their games. It keeps all of your content with you and stops people from games sharing.

I can't see why everyone would want multiple account anyway? I share a single PSN / XBL account with my family and if it REALLY bothers people that much then is it too much hassle for a friend or relative to buy a cheap 4gb / 8gb memory stick of their own?

I would only have a problem if SONY didn't include a memory stick with the system, especially if you need it for games.