Edit: I dont know anything about the Vita.
As far as the Ps3 goes, as long as both accounts are on the same system, yes you can, assuming there are no age restrictions in place.
No. The Vita can only be activated under one PSN account at a time, and only content purchased by the activated account can be downloaded or transferred to the Vita. The only way to change the PSN account associated with a Vita is to restore the console to factory state, which deletes everything. You can't sign in to PSN on the Vita using any account except for the one that was used for initial configuration.