After a Sony representative told Wired that you could use a different memory stick on your PS Vita to access a different PSN account, Sony has backtracked and said that only one user account can be used, even if you have multiple memory cards.
In an update to Wired, a (different) Sony representative said:
[The] PSN account is tied to the hardware and the memory card, not just the card, which means that if a second person is using your Vita, it’s not just a case of switching out memory cards, it’s clearing out all of your saved data on the Vita itself when you do the factory reset.
In other words, PlayStation Vita is intended to be played by only one user.
This means that, to share your Vita with a friend or relative who has a different account, or to access a store from another region, you’ll have to factory reset your handheld – deleting all data.
We’ll be getting our hands on a Japanese PlayStation Vita when it launches, and will be sure to let you know if there are any loopholes or tricks to get around the account lock.