Qbittorrent: you can bind the application with a network interface and ensure all the connexion will use your vpn.
bonus: you can use it as a server (without any graphical interface) and manage the torrent with your browser. This way, you can create a torrentbox on a dedicated computer.
You can check this project : https://github.com/showmewebcam/showmewebcam
It’s uses buildroot and it’s easy to change the sources for your need (I’ve added the wifi support)