The vault is a safe place where you can store your funds and 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 funds without having to worry about transaction fees and the delays involved with the blockchain.
Any access to the vault will require confirmation of the security measures opted into on your account, i.e. password, verified email, 2FA, etc.
Funds can stay in your vault for as long as you would like, as there are no limitations in that way.