In the same boat as you (with an Ivacy purchase too).
I've tried qBitTorrent, Deluge, DownloadCenter and they all 'work' but none of them are easily configured to force traffic through the VPN.
(Please someone prove me wrong!)
I've even tinkered with Docker images of torrent clients ... but i've struggled with setup ... that's my failing
Combine that with the fact Asustor's OS doesn't re-connect to the VPN properly means i've given up using my NAS for this.
If i can't easily configure and TRUST it i'm staying clear.