In the most recent firmware update to the PlayStation Vita, Sony managed to sneak in a bit of code that now locks PSV memory cards to single PlayStation Network accounts. Prior to the 1.80 update, PSV owners with multiple PSN accounts could swap their accounts relatively easily.
If you have multiple PSN accounts with different titles tied to each, which is likely considering some PS Stores get region-specific games, then you’ll need to purchase separate memory cards for each PSN account. Alternatively, you can allow your PSV system to format your memory card so that it can be tied to a different PSN account. Yeah, not really an option is it?
Basically, Sony has made PSV owners’ lives that much more complicated, and they’ve done so in order to protect themselves from people sharing games now that PSone classics have become available on the system. Too bad those memory cards come with a premium price tag.