hi…
i have avast internet security 2015 installed. How can i set the firewall in a way that it only allows internet access when the pptp vpn connection is active, and block internet access when it fails or disconnected?
You can’t.
You either allow something or deny something in a firewall.
There are no “if then” settings in a firewall, unless you get yourself a real (hardware) that you can fully program yourself.
It will cost you several $1000’s