There are two recommended approaches for monitoring VPN or WAN link availability:
Monitor an element or service on the other end of the link
The most straight forward method is to simply monitor a service or element that resides on the other end of the WAN or VPN gateway. If the service or element goes offline, up.time will trigger alerts indicating the outage.
Monitor the VPN or WAN gateway device
Most network devices include native SNMP MIBs that can be used to poll the device about the status of each known link. Using the up.time SNMP Poller service monitor, you can select the OID that represents the status of your link and monitor it for any changes.