The issue with (free) VPNs is that the IP is basically shared. Although it might be uncommon for the same two (or more) users to be using the same VPN service in the same casino, it's not an impossible scenario and could potentially explain why some users might be accused of multi-accounting, even in VPN-friendly casinos.
An IP address alone isn't enough to detect multi-accounting. Factors like screen size, device ID, browser, platform/OS, user agent, and user activity patterns are more commonly used for that purpose. So using a free VPN isn't really a problem in terms of multi-account detection.
The main concern with free VPNs is usually security and a poor browsing experience, as they tend to be slow and unreliable.
It doesn't really and a lot must be considered before an account can be declared as connected and as a matter of fact we have what we call public VPN policy and this warning is on the platform, you get a massage warning that you are using a public VPN and at that other factors is in place that can affect or not affect the gambler activities on the casino.
Most user's that are connected have some level of issues with the platform, most especially those that carry out illegal activities with their alts in the casino, all this are the main reasons why a casino can connect two accounts.