There may be another way to do this, but adjusting the Interface Metric under Advanced as seen in the following screenshots:
Click Properties
Click Advanced
Adjust the Interface Metric to, eg, 10
After doing this, adjust your WiFI and/or Ethernet Interface Metrics likewise (I have Ethernet set to 1, WiFi to 5, and the VPN to 10). This [should] ensure that only "appropriate" traffic goes over the VPN.