How would I deactivate PS Vita (3G/WiFi) without knowing anything about previous holder? I live in UK, and I suspect that system was sold with O2 contract.
If the system is signed in you should be able to just go to the Settings app, then Playstation Network in the menu, then System Activation, and pick Game and Video/Music and select Deactivate on both options. Then sign out and sign in with your own PSN account or create a new one. You'll lose access to the downloadable games bought by the previous owner, but that's just how it works.
If you can't sign in, go to Format, Restore the PS Vita System and restore it; note this will erase * all data* on the Vita, save games/pictures/whatever. Use the Content Manager to back up your saves first if you have any.If you haven't played it at all and there's nothing on the system you care about, you might just want to start with the format. You'll need to update your Vita again after the format (I think...I haven't actually reset mine) then sign in and add your games.