The Vita is not region locked, however obviously we can't buy PAL or Japan content from our US PSN accounts. However, one would assume imported game cards will still work. I don't think you have to worry about that.
If you import a game physically, it will work regardless of your account. However, if that game requires an online pass to play online, it effectively means its impossible to play that game online unless your PSN account matches the region of where you purchased the game from, so you may have to constantly format your vita to play different games online.
Since Sony is adopting online passes with open arms, almost all games on the vita will eventually do this.