What VPN rules are required to kill traffic if VPN is down?

[archedraft]

I want to do the following:
Alias: VPN_only -> If VPN goes down make sure no internet connection
Alias: VPN_ISP -> Uses VPN unless it is down and then uses ISP
All other connections go directly to ISP.

What specific rules are need to accomplish this?

[kesawi]

Have a look at /index.php?topic=106305.0, particularly sections 9 and 10.