Who would like to have the vaults in their ship instead of the tower? This way you can get the weapoin you need when you are in orbit instead of going through the load times of getting to the tower simply for getting weapons you stored. Maybe even make vaults bigger with upgraded ships. And if there is an issue with 3 characters and ship vaults you could just make vaults on the ships that link to the central vault. Like a transfer interface. As long as you can access from orbit.