The PSP and 3DS are currently restricted to one online account per system. However, it appears the Vita will introduce the ability to have multiple users on one handheld... with one huge caveat.
A PlayStation representative confirmed that PSN IDs are not tied to a system. Rather, they're tied to the system's proprietary memory cards. If you wanted to swap between PSN IDs, all you would need to do is swap out the memory stick.
"Your PSN ID is bonded to your memory card and your memory card is bonded to your Vita," Sony's Crystal MacKenzie told Wired. She clarified that a single PSN user is also allowed to use multiple memory cards, if you need to carry around multiple memory cards for some reason.
The proprietary memory cards have been designed in an attempt to curb the piracy Sony experienced during the PSP generation.
If you no longer want to use your Vita memory card, and want to give it to someone else, you'll have to perform a factory reset in order to wipe it of your personal data. Remember that, before trading those cards into GameStop!
Sony has yet to officially announce the prices of Vita memory cards, but initially leaked retail prices were not met favorably.