I found this tip in the comments on a VPN article:

Your default gateway is determined by the order of your network interfaces in the Network pane of System Preferences. Whichever interface is at the top of the list will be the default gateway. If that interface isn't availble, the next in the list will be the default gateway, and so forth. What you want to do is be sure that the interface used for your Internet connection is above the one for PPTP.

To change the order, select "Network Port Configurations" from the Show drop down menu. This will display a screen which allows you to drag and drop your interfaces into the desired order.

It worked for me. What a relief.