The vault is a separate balance from your standard wallet balance.
You can find the Vault by heading to the profile dropdown on the LHS bar or the top right of the page.
Clicking on the 'Vault' button will bring the modal below up.
Here, players can choose to:
Transfer in or out of the vault
Their preferred currency
The amount they'd like to transfer.
Any funds within the vault cannot be used to bet on games - this allows players to effectively separate their gaming balances.
Players can transfer between their wallet and vault as much as they'd like.
Vault transactions can be found in the transactions page under the 'Vault' tab here.