I have found that Zabbix is indispensable in monitoring infrastructure, however if you followed my previous articles on OpenVPN that setup in particular had a couple unique challenges for monitoring.
If you want to monitor the OpenVPN log file there is a rather complex regex query to walk the status log and count the number of clients connected that returs a number to Zabbix you can set a trigger on. If that sounds like what you want check out this post.
If you want Zabbix to simply ping devices connected over the VPN (like your IP field controllers, jaces etc) to verify they have connectivity to the server that’s also a problem because if you are using my writeup the BMS VPN LAN connecting over OpenVPN does not route to the servers LAN where Zabbix lives. But the Zabbix Agent on the BMS server running the OpenVPN server can see those devices so we can ask it to ping those on our behalf. If that sounds like what you want check out this post.