How resource intensive / business focused is your product?
Lots of people use vpns for work or privacy, I think that method would have a lot of false positives. In cases where trial account usage costs you very little or nothing I'd err on the side of not blocking users.
People don’t use public VPNs for work. The tunnel terminates in the same network that the physical office is on. And using a public VPN from a work machine for privacy is a very bad idea. That’s grounds for termination if they catch you and possibly an even worse issue if the company ends up under investigation for any reason. They’re going to take a very close look at anything of yours they can get access to since it won’t have ended up in normal audit logs.
What do you mean by public VPN? Do you mean free proxies? I'm talking about paid private vpns like expressvpn. The p in vpn stands for private so that's a bit redundant. Your traffic is still encrypted, these vpns can't run a MITM attack on you.
Lots of people use vpns for work or privacy, I think that method would have a lot of false positives. In cases where trial account usage costs you very little or nothing I'd err on the side of not blocking users.