If the VPN connection goes down, VDSM including any package running under it will default back to the default LAN adapter. If that LAN adapter has configured gateway parameter, the traffic will continue forward outside VPN.
Multiple Gateways setting is by default turned off, have you activated it for a specific reason? Guessing it is needed in order to connect to that VDSM instance from the outside while the VPN connection is active?
Bottom line, a "kill-switch" will not work out of the box. Try and remove the gateway parameter on your main VDSM adapter, and connect via VPN. Then, download and terminate the VPN. See if the traffic will stop in that case.
Thanks for the quick reply!
I did not activate the multiple gateways setting. I thought I deactivated it, but in any case the tick box is "unchecked". I am still able to connect to the VDSM remotely and locally with that setting unchecked. What does that setting do if it doesn't force VDSM to use only the default gateway?
I believe if I remove the gateway parameter for the main VDSM (LAN adapter) I will lose all connectivity. The VPN connection is remote through a third party (Windscribe) so it relies on the LAN adapter to provide internet connection if I understand it correctly.
Perhaps I need to configure the VPN on my router to direct all VDSM IP traffic through the VPN? I'll have to look for info about how to do that with my router (Orbi RBR50) and see if router supports such a configuration.