PlayStation Vita will allow you to use different PSN account on the same device by just swapping the memory card, this is because PSN accounts for PS Vita are tied to a memory card, not the system like the PS3 does. This idea may sound good for many, but it has its downsides. Before adding using another PSN ID, you’ll have to restore the system to default settings first. Memory cards for PS Vita are also over priced as well.
This is what Sony associate brand marketing manager Crystal MacKenzie told Wired.com:
“Your PSN ID is bonded to your memory card and your memory card is bonded to your Vita,” MacKenzie said. “So if you wanted to change different PSN users but use the same memory card, you would need to go factory reset.”
Customers will be able to swap cards to use different PSN IDs on the same system, MacKenzie said. If you have more than one memory card, you can register a different PlayStation Network account on each.
“You can also use them both on the same PSN account as well,” she said.
Source – Wired.com