How to monitor a vpn server?

Has anyone used GTM or LTM to monitor a vpn server? I'm trying to monitor the IKE port 500 (udp) from GTM, but GTM requires a valid response to mark the server up and the vpn server won't respond to non-IKE requests.

 

I've seen a dns monitor script, but that would be an invalid request as well as determined by the VPN server.