Most VPN’s come with a kill switch to help prevent transfers if they close or crash. However, if you only really mask your torrent usage then it’s an idea to bind the IP and network interface to that of your VPN. This will prevent any transfers from occurring unless you are connected to your VPN.
(You can get the VPN interface name and VPN IP from Control Panel, Network Connections under Windows, It will more than likely be listed as TAP Adapter, double click and then select details to get it’s IPv4 address)
in qBittorrent go to Tools, Options and then Advanced.
Under Network interface, select your VPN and Apply
Under optional IP to bind to, select your VPN Network IP and click Apply.
Restart qBittorrent and check that things transfer correctly on the VPN, disconnect your VPN and make sure that the torrent traffic stops.