I use expressVPN. The pc has ethernet and wifi access to the same LAN.
When I activate expressVPN, it only uses the ethernet. Why does it pick that one? There is no option in the application for selecting which adapter.
Which adapter a pc uses will depend on its routing table.
If you find out what IP addresses express vpn connects to you can set up a route with a different metric to use your wifi or other adapter.
By default on a sensible OS (sometimes windows may differ due to drivers) Ethernet is “better” than wifi so it will have a lower metric (higher priority) and be used in preference if both are active.