The vault is a safe place where you can store your coins and which can be accessed by navigating to Account > Vault from the main user menu.
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. There are absolutely no time limitations or restrictions.