I think to monitor your VPN , you can open your VMS and go to VPN/Security Management Solution > Monitoring Center > Performance Monitor
Then click on the Monitor tab and selete 'remote access VPN' or 'Site-to-Site' VPN ..etc
For detailed instruction on how to add your VPN, please click on the help link on the top right corner , it will provide with you detailed information.
Topics in Contents tab in the help regarding VPN are "monitoring Remote
Access VPN services", "Monitoring Site-to-Site VPN services
Hope this is helpful