According to the recent reports, Sony’s upcoming handheld PS Vita was condemned a lot by the fans on the issue of only one account handling by the same device. Sony has clarified the misconception now. Speaking with Wired, Crystal MacKenzie, the Associate Brand Marketing Manager of Sony told that you can use two accounts on the same device (without factory resetting it) using multiple memory cards.
Your PSN ID is bonded to your memory card and your memory card is bonded to your Vita. So if you wanted to change different PSN users but use the same memory card, you would need to go factory reset.
She also told that one can use the same PSN account on multiple cards. So now you can calm down a bit but you will have to pay extra anyways if you plan to share the device with other family members or friends. Better than buying a new device, right?