Vanguard offers a variety of account closure options, including online closures and phone closings.
To close your Vanguard 401k account, you will need to provide your employer with a request for closures. The employer will then send you a closure notice. You must follow the closure instructions included in the notice to close your account.
Yes, you can cancel your 401k and cash out while still employed. However, you will have to wait until after your employer contributions are fully paid off to do so.