Terms and conditions of poker sites disallow the use of VPNs for the purpose of grinding while in a restricted country, but the sites have no issues with regular grinders taking advantage of the vital security benefit provided by another layer of encryption.
Yes, it's possible for poker sites to detect VPN usage.\n\n Just like Netflix and other online platforms, poker pages might see a surge of identical IP addresses, which happens when lots of people use the same VPN server to play at the same time.