Idle timer to trigger vlock?

[Paul Reilly]

Could someone point me in the right direction to understand how the TMOUT
variable is monitored, and logs out the session when it's expired?
I'm looking for a way to lock a users command session with vlock (or
similar) when they are idle for a user-defined time.
At the moment, I use TMOUT=7200 but this logs them out, which is
inconvenient. I just want to lock their session, requiring them to enter
their password to unlock the session, similar to how xscreensaver would do
if they were using a GUI.

Thanks
Paul