Since you’re running on Linux you don’t need extra software like some suggested.
Systemd can be used to restrict a program networking to your VPN. The following is an example of how to it for Transmission and you can adapt it to any other service. It boils down to overriding the default daemon unit by using the following command:
<span style="color:#323232;">[Service]
</span><span style="color:#323232;">IPAddressDeny=any
</span><span style="color:#323232;">IPAddressAllow=10.0.0.1 # --> your VPN IP here
</span>
Another systemd option, might be to restrict it to a single network interface:
<span style="color:#323232;">[Service]
</span><span style="color:#323232;">RestrictNetworkInterfaces=wg0 # --> your VPN interface
</span>
Save the file and run systemctl daemon-reload followed by systemctl restart transmission-daemon.service and it should be applied.