Be sure you have only one network adapter enabled, example: if you are using ethernet, be sure wireless is disabled. If using wireless be sure no ethernet cable is connected, or disable the adapter in networking and sharing center > change adapter settings.
Open command prompt and do a command:
note the Interface ip address on the 0.0.0.0 line in the "Active Routes" section , lets just say it is 192.168.0.1
Connect to your VPN
Note you will see a new ip route for the vpn (second 0.0.0.0. line), note that Interface new route ip
Now kill the original non vpn route with this command
route delete 0.0.0.0 192.168.0.1
then do another
you will only see your vpn route now, and if your VPN line drops, you lose that route, so there are no more 0.0.0.0 routes and your external connection will be immediately cut off. If you want to access the internet you'll need to add the original route back with this command:
route add 0.0.0.0 mask 0.0.0.0 192.168.0.1 IF 8
The "IF 8" means "interface 8". The number 8 may be different on your computer, look at the output of netstat -r to get the correct number of your interface in the Interface List
All that being said I would disable ipv6, I have done this with no issues after doing so.
disable ipv6 command:
netsh interface teredo set state disabled
Enable ipv6 command:
netsh interface teredo set state default