A correct VPN will make it look like you're just sending garbage traffic to/from one destination, the outside looking in traffic pattern is completely different than a torrent directly (which is many to one + one to many)
Does it really look like garbage from the outside? My impression or understanding was that you could tease out those details (they are torrenting) but not inspect the packets directly (what are they torrenting?)
A proper VPN will completely encapsulate your layer 7 data, so you should not be able to tease out the fact that they are torrenting - it should more or less be an opaque stream.
With some protocols you can identify that they are sending VPN traffic to and from a destination, but that should be it, otherwise something has gone horribly wrong in a dangerous way.
Depending on the VPN protocol, you can still fingerprint by size - as I understand it, bandwidth use patterns are actually enough to distinguish things like "streaming video" from "BitTorrent traffic" from "web browsing". IIRC, ex. TOR does a bucket-filling approach to fight this (something along the lines of trying to wait until you've got X bytes to send together to smooth out the packet size, or even inserting garbage to pad out your use).
