Both work quite differently. TOR routes you over several layers, obscures your IP and changes the IPs around occasionally so you can’t be tracked.
With Bittorrent you want lasting connections to other peers to be able to receive and send all the data. This doesn’t align with the ever changing IPs and stuff.
A VPN gives you one IP that you can have for hours.
A VPN supports UDP connections, TOR doesn’t.
Connecting your Bittorrent client to the Socks-Proxy of a TOR client is a different setup than it just sending normal packets through a VPN tunnel.
TOR is slow (by design), a VPN is fast.
If your client or something leaks your IP it happens anyways, if you route it over one node or seven. All the extra energy is just wasted.
And bittorrent puts even more strain on the TOR network the way it works. Making it slower for anybody else. And (ab)using the resources volunteers provide. (And which are meant for better use-cases.)