20 should be fine. you can already hold a good amount of weapons on you. I don't think my inventory was ever near full in the beta. The vault's primary purpose is a means to trade loot between characters, so its not like you were meant to store every single thing you weren't using. Perhaps just some stuff you can't use yet. As as for those legendary and exotics, you will be trashing quite a few anyways. You need to disassemble items in order to acquire upgrade materials for items of the same rarity. Meaning if you want to obtain all upgrades for your favorite exotic, you are going to have to part with a few other exotics to do so (unless you want to spend several hours collecting materials such as spinmetal to trade in for it.) The game is putting a lot of emphasis on choosing one good weapon to use, and if you only end up using one thing, there is no need to really hold onto all of the rest. Just get comfy with one weapon and armor set, upgrade when possible, and break down the items you never intend to use again. You may never even have to use your vault.