Ok in case you haven't heard, let me explain this. When you farming the chests multiple times, you get multiple Special ammo synthesis. Which stacks up in your bags, and fills out your inventory and vault within a few days of farming chests. Eventually you get so many that you don't have anymore room in the vault for materials. Hope that explained that for you. :) It really depends how hard you farm these chests. For people that are able to pick it up 3 or 4 times, they can get full vault literally in a day or two.

I just use my secondary now when I'm roaming and use synth until I'm down to one stack.