If you're actually using a VPN (as opposed to a SOCKS proxy) then what they can certainly identify is that you're moving VPN traffic, the volume of traffic each way, and probably that it's torrent traffic (gaming, streaming, surfing will have very different usage patterns and much lower uploads most likely). For torrent traffic, they can also tell whether you're seeding a lot, leeching/downloading without seeding, or a mixture.
This makes sense if you think about the kinds of traffic you'd expect from the most common uses. There's a pretty limited set of categories that most Internet traffic can be grouped into, and they all will have pretty distinct and identifiable patterns - commercial streaming will be slow and steady, downloads will be bursty, VoIP will be small streams, etc.
For sophisticated enough systems or determined investigators with enough raw material to examine, they may even be able to get a decent guess at what sites you're using (even via a VPN) if you're only doing one activity at a time. If they can identify that almost every video view on YouTube starts with a particular traffic pattern of sets of blocks of data of roughly identifiable sizes and that video views on Netflix, Vimeo, Hulu, YouPorn, or whereever all have different patterns, they can approximate which sites you're visiting - particularly given enough data over time.