It’s against Steams Terms of Service to use a VPN for anything, even if you’re doing nothing but staring at your Library while it’s running. So yes, it’s possible to get banned, but unlikely because they don’t enforce it in that manner.
The one thing you shouldn’t do while using a VPN on Steam is purchase from their store. If you’re going to buy something on Steam (This does not include buying things in-game from the TP Shop), then make sure to turn the VPN off first.