How to determine which traffic goes over the PPTP VPN?
Have you tried using the route command to declare a static route?

For instance, I use the following with my VPN:

sudo route -v -net 10.41.0.0 -netmask 255.255.0.0 -interface ppp0


This tells any traffic going to 10.41.x.x to go over the VPN interface (ppp0).