Here's what would solve vault space: Bring up a 'pop up' menu that you can access in orbit, or during gameplay, so that you can cycle the weapons you want to use for that particular mission. That would eliminate the need to have the same weapon for each guardian, and curb 'collector whores' like me from maxing out all my weapon/vault space. This way, you'd only need one Vex,Ghorn, Fatebringer etc...