Transmission

If I download a torrent, it loads into Transmission and starts downloading all down through the VPN. If I quit Transmission and reopen Transmission later to complete the download - but forget to start my VPN and it is now through my regular ISP am I now vulnerable via my ISP?

Yeah, that would leave you vulnerable. You need to switch to a VPN with a kill switch that stops all traffic from entering or leaving your device if you are disconnected from a VPN server location. For example, this is the one I use:
https://www.expressvpn.com/support/trou ... work-lock/

Thanks for the information. I do use ExpressVPN. I'm new to the whole VPN thing.

if you switch it on, you'd be fine in the future!