Personally, I have a reason for having no armor in my vault but lots of weapons. Armor is not shared between classes. So a Hunter can only wear Hunter armor. The reason why I use the vault at all, is mainly not to store the stuff I have but to have easy access with all characters. I know, now you have the app you can switch weapons quicker but I still like to have a central place to put all my weapons and not having to look for them on my characters etc. So, I just keep all the armor that I use on the respective class, while I switch around weapons between my characters all the time.