Borderlands 3 Vault Card Chest. A piece of gear from the weekly loot pool, a cosmetic item from that vault card, eridium, a vault card key (which are rare), or a diamond key. Go to the vault card tab in your inventory, at the bottom it will say you can open one from there by pressing x
These special chests contain a variety of items including the rare diamond key. As you complete the given challenges, you gain xp; Use the key to open it, and the three walls behind it will open, allowing you access to all the cool guns, grenades, and shields.