there are some ways to get this to work:
1.create a node with the vpn ip address and icmp poll against that address for up/down stats.
2. setup SNMP Trap alerting; the vpn device will trap on a tunnel going down, and you could use the trap viewer to generate and email based on the trap from with in npm.
3.or you use the Universal Device Poller to monitor these in NPM with the info below: