Update: A Sony representative confirmed PSN accounts are tied to the Vita console and memory cards. Only one account can be used per system so if you want to switch to another account, you must to reset your Vita to factory settings and format memory cards.
The cost of Vita's pricey memory cards has already made many wallets lighter but for those who manage multiple PSN accounts, things just got even more expensive.
Sony has confirmed that PSN accounts will be tied to memory cards. If you want to use more than one PSN account on your Vita, you'll need to buy more than one memory card.
The only way to use multiple accounts with one card would be to reformat the card. Of course, this method would be very time consuming as you would have to constantly redownload data.
Folks with large collections linked to their account won't have to worry since they can store their games on multiple cards.