Below are two scripts, one for Viscosity for Mac, and the other for Windows that you can implement into Viscosity to close any program you specify upon VPN disconnection. They are very easy to implement. It should take you no longer than 30 seconds. The 5 step instruction process is included in the link. I hope this helps you out!
Link for Mac:
Link for Windows:
SIDE NOTE-For Windows that don't use Viscosity there is another VPN killswitch that is free and works with every VPN. It is called VPNCheck.