Can't save anymore in Fallout:New Vegas for the PS3
You should check your hard drive space, which you can do in the system settings on the XMB. Then you should try to free up some space. This you can do by deleting downloaded content, and installed game data.
In any case, you can also try deleting save games. Go to the games menu, go up, find your saves, find the fallout folder, see the saves. Press triangle, do to delete. Do this as often as you like.