It’s not clear what this means,
If the openvpn service crashes (which in my experience rarely happens), then you can modify systemd to restart it automatically. See here for details.
Otherwise, you need to do something yourself, either using iptables or perhaps a shell script to modify transmission’s settings.json parameter bind-address-ipv4 to the tunnel’s IP address. Alternatively, a shell script to detect that the tunnel is down and then stop transmission.