eth0 is not listening on port 80 when PPTP is not connected
Solution 1:
Try configuring the nginx vhost to listen to your server's ip address no port 80 and not on all interfaces. And by that I mean that, rather than using:
listen 80;
in nginx, try using:
listen server-ip 80;
This way your nginx server only binds to port 80 to that specific ip address and not on all interfaces.