The Vaults are storage rooms that can be found under every Fortress Island in The Sea of Thieves. While the Vaults are usually empty and open, they will hide high value Treasure behind a locked door as soon as a Fort becomes active. Active Forts are indicated by a large Skull shaped cloud in the sky. These locked Vaults can be unlocked with Skull Shaped medallion Keys, which are dropped by the final boss encounters of Active Forts.
Skeleton Fort Vault
The Vaults of all Forts remain open by default. As soon as a Fort becomes an active Skeleton Fort (indicated by a green-eyed Skull Cloud), the Vault will fill up with around ~20,000 Gold's worth of Treasure and become locked. In order to open the Vault, a Crew must disperse of all the waves of Skeletons and defeat the Skeleton Lord of the final wave. Once the Lord is dead, it will drop a green Vault Key, which can be used to unlock the Treasure Vault of that Fort. The Vault itself contains large amounts of loot which will require multiple trips to ferry it to your ship. The most notable items that are guaranteed to be found in a Fortress Vault are a Stronghold Chest, a Stronghold Skull, a Crate of Ancient Bone Dust and a Stronghold Gunpowder Barrel. The Molten Sands Fortress vault in the Devil's Roar replaces all of the regular loot with Ashen counterparts and a Chest of Rage.
Fort of the Damned Vault
The Vault of the special Fort of the Damned Fortress acts near identically to its regular Skeleton Fort counterpart. The Vault will remain open as long as the Fort itself is inactive, but once the Fort of the Damned Ritual is complete and a Fort of the Damned Raid is started (indicated by a red-eyed Skull Cloud), the Vault will spawn Treasure and become locked. Any players standing in the Vault will get teleported out once activated. The Vault can only be opened with a Fort of the Damned Key, a Red version of the regular Stronghold Key, which is dropped by the final Skeleton Lord boss of the Fort Raid, the Ghost of Graymarrow. Due to the increased difficulty of the Fort, the Treasure is also more valuable, containing around ~50,000 Gold's worth of Items: 1 Chest of Legends, 2 Reaper’s Chests or 2 Reaper's Bounty, 2 Stronghold Chests, 2 Crate of Ancient Bone Dust, 4 Stronghold Skulls, 4 Stronghold Gunpowder Barrels, 5 Gunpowder Barrels and 4 Mermaid Gems.