vlock [ -a,--all ] [ -c,--current ] [ -h,--help ] [ -v,--version ]
By default, only the current VC (virtual console) is locked. With the -a,-all option all VCs are locked. The locked VCs cannot be unlocked without the invoker's password or the root password. The root password will always be able to unlock any or all sessions. And, for the paranoid, vlock makes it a trying experience for those attempting to guess the password, so unauthorized access to session(s) is highly unlikely.
Please note that it is entirely possible to completely lock yourself out of the console with the -a,--all option if you cannot remember your password! Unless you are able to kill vlock by logging in remotely via a serial terminal or network, a hard reset is the only method of ``unlocking'' the display.
vlock works for console sessions primarily. However, there is support for trying to lock non-console sessions as well, but that support has not been well tested.