As NeoGAF discovered over the weekend, the PS Vita allows only one PlayStation Network account per portable.
Whatever does that mean?
In a FAQ from a Vita hands-on event with the general public, the question regarding the number of accounts per system was answered as such (translation via GAF's mehdi_san): "Only 1 account. If you want to to use a different account, you need to format the system to factory settings."
Via Twitter, Sony's Brad Douglas, Sony Japan's Third-Party Relations man, clarified the issue, pointing out that the PlayStation Portable also limits players to one account. "The only difference is that Vita requires you to do a factory reset when logging into a new acc[ount]."
The PlayStation Portable, however, does not.
The reason why many PlayStation users have multiple accounts is so that they can play region specific digital titles available in, for example, the Japanese PSN Store as well as content available in their home country.