You can code a custom script that uses the IP address defined in their product and check the status of the VPS and services. You can also use the IP address against your monitoring software, service etc. We pull the information from our Nagios monitoring server and parse it on a per client basis. It's quite a complex setup but doing a simple check on their IP will work for now.
I strongly recommend you show the node status as well, as this will be very helpful to the client if their server does down. You can also look at showing the node memory and cpu load via remote nagios / php script etc
WHMCS allows you to show information based on login status etc, see the documentation.