So… the first Vault was a prison for an extra-dimensional god of destruction made flesh. The second Vault housed a living Eridian superweapon.
What actually was in this alleged third Vault found by Tannis and dubbed the Badass Crater of Badassitude by Mr. Torgue ("…that’s not what the area is called…!")
Surely not just a lootsplosion of underwhelming weapons.
Maybe just a cleaning supplies closet for the Eridians or something. I dunno.