Use docker with something like gluetun and run transmission docker sharing gluetun network.

I know you're asking about transmission, but.. Qbittorrent has an option in the GUI that allows you to specify what network interface it uses to communicate through. Therefore if your VPN isn't up, it won't use your normal network. And if your VPN drops out, it also won't use your normal network. Just another option in case you're interested.

I use qbittorrent in a docker container which sits on the network stack of another wireguard container connected to mullvad.

I’m using this if it helps and if you want to use docker : https://hub.docker.com/r/haugene/transmission-openvpn

I found this: https://askubuntu.com/questions/37412/how-can-i-ensure-transmission-traffic-uses-a-vpn#205510 Hope it helps

Also, VPN usually have a killswitch feature where it turn off the connection if the VPN stops working.