Vault is a feature that works as a bank - you can store your Stake Cash and Gold Coins there and separate them from the tokens you wish to play with or if you want to save them for later usage. It is an inactive part of your balance and for as long as the tokes are in your Vault, they cannot be used for playing.
In order to navigate your Vault and store tokens there, go to Account icon > Vault > Store, choose the tokens you wish to move there and input the amount.
If you already have tokens in your Vault, but you wish to move them back to your balance, go to Account icon > Vault > Retrieve and fill in the blank fields.
The purpose of the Vault is to provide a safe and convenient place to store your coins.
Any access to the vault will require confirmation of the security measures opted on your account, i.e. password, verified email, 2FA, etc.
Coins can stay in your vault for as long as you would like, so there's no limitation in that way.