Is fast user switching in Lion disabling the network connection for the accounts that aren't active?
Not really - Lion is now able to kill processes that appear to be idle and unused. Fast user switching adds more load on memory and processor (due to the simple fact that more is now running), increasing the chance this will trigger.

Also, the changes to allow multiple simultaneous logins may have changed the API which will need the VPN client to request to stay connected or otherwise start itself outside the user space to survive a switch in Lion.