Don't be simple. This can reveal people that are using proxies like Tor and VPNs, and it's already being exploited by trackers:
Looks like the Tor bundle disabled WebRTC about two years ago [1] [2]. I think a VPN user would expect the WebRTC connection to be routed via a VPN, as well, but I'm still figuring out how those work, exactly :)
0. PPTP exposes that you're on VPN and your computer's IP http://i.imgur.com/mKKfjj7.png, as does L2TP http://i.imgur.com/C68HvSN.png, while OpenVPN only exposes that you're (probably) on a VPN: http://i.imgur.com/IVQkwsd.png.