The Treasure Vault is a room in northern Valka Castle. As its name suggests, it is an area rich in salvage items.
How to Unlock it
This room will not be a crucial part to the story's overall plot, but the player will find its entrance, a door with a hole in it, at coordinates B-1. Focusing on the door using the A button will prompt the player character to notice that, if they were small enough, someone could fit through the hole in the door.
Diving with Oceana will allow the player to unlock the door. After the player has noticed the hole in the door and its size, they can then talk to Oceana when they're diving with her. She'll ask the player if they want her to go into the room and unlock the door from the other side. Saying yes will prompt a short cutscene showing Oceana unlocking the Treasure Vault.
As well as the Imperial Treasure, the Servant's Ring is found here. It is not found by using the multisensor; rather, the player can see one of the sparkles representing an interactive or quest item, and the Servant's Ring is found under said glow. This ring is crucial to finding the Golden Catfish.
False Killer Whale
The player will need to open the Treasure Vault to befriend the false killer whale in Valka Castle. When the player has gotten to a certain point in Be the Best Trainer, Finley asks them to befriend an False Killer Whale with an X-shaped scar on its forehead.
At one point in the mission, the whale swims into the treasure vault. After the player encounters it here, it swims away, only to show up in the Mermaid's Ballroom .