Treasure Vault is one of the Chamber Dungeons found in Link's Awakening.
Treasure Vault is the seventh Chamber Dungeon unlocked in the game. It one of four of the 'Level 2' Chambers.
The chamber dungeon is unlocked after Link completes the Bombs Away! as well as Catfish's Maw in the main quest. The dungeon has eight tiles in the center that all require Treasure Chests. Link will need to fill these with treasure chest tiles, but at least one of them will also need a staircase to connect to a tile outside of the square, since there needs to be a dungeon entrance and boss room. Link is able to optimally complete this dungeon by placing 11-tiles, 3-outside of the vault and then the 8-inside.
Link will earn a Chamber Stone for arranging and then completing the dungeon. Completing the dungeon will also unlock the Passage Way Central dungeon.
The vault needs treasure. Stuff those tiles that have treasure-chest marks with chambers that have chests. I don't gotta tell you how to bridge the gap, right?