The system is a PC which has an ethernet port as well as WIFI. Normally it will only be using the wired network (which is not connected to the outside world at all), but from time to time there is a need to connect to WIFI and a VPN over the WIFI - without interfering with the normal operation of the wired network, and that is where my eyes begin to cross.
The OS is Ubuntu 22.04, and the VPN uses L2TP; I've had this work when the only networking is WIFI, but how can I limit the VPN to use only the WIFI?