Vault Keys are a special dungeon key which can be acquired from the mysterious trader at the Outpost. After completing the game's main story quests he'll offer a quest, The Final Frontier, after which the key will be available for trade from him. Each key opens an Ancient Vaults dungeon at an Ancient Gateway.
This quest will task players with trading him for a Vault Key. Speaking to him again will show what items he requests in exchange for a key.
For each key he'll require a number of elemental drops, found by killing monsters or harvesting fluffalo. Specifically, he may ask for bundles of 10 Scorched Cores, Cryonic Extracts, Static Cells, Hardened Carapaces, Living Roots, Venom Samples, pieces of Phase Matter, Sharpened Claws, or Sticks of RAM; alternately, he may ask for bundles of 40 Refined Aegisalt, Refined Ferozium or Refined Violium. Once the key has been collected, players must go to an Ancient Gateway and enter the Ancient Vaults dungeon.
After opening the gate, the key will be consumed. The dungeon portal will remain opened for 30 minutes, after that it will close and another key will be required to enter a new vault.