One of the advantages to using this system for user accounts is that you can take advantage of the UNIX user administration commands. Since each client is logged in as a user, you can use standard methods to get user statistics. The following are a few commands that I like to use to see what all is going on.
Prints the users currently logged in, as well as when they logged in, from where (name or IP), and on which port.
This command prints a more extensive listing of who is currently logged in. It also tells you uptime and load averages for the system. It also lists the user's current process (which should be -pppd for VPN clients) as well as idle time, and current CPU usage for all processes as well as the current process. Read the w man page for more info.
This lists the login history for the specified user, or for all users if a username is not provided. It's most useful for finding out how well the tunnels are running as it prints the length of time that the user was logged in, or states that the user is still logged in. I should warn you that on a system that has been up a long time, this list can grow extremely long. Pipe is through grep or head to find out exactly what you want to know.
You can also control which users are allowed to connect by modifying the /home/vpn-users/.ssh/authorized_keys file. If you remove the user's public key line from this file, they won't be able to log in.