If you use a VPN from a restricted area, you can not login.
When you use a VPN, you logically do not show your "real" IP!
Therefore, what you say does not make any sense at all!
You can login because you are already using different IP which casino allowed. This is the real use of VPN on casino by hiding your real IP and use different country so that you can play as different nationality.
This is a case to case basis depending on the ToS of the casino about VPN.IIRC Stake have a mirror link so that restricted customers can access the casino without the use of VPN.
To my understanding, I always assumed those mirror links were interned to access to Stake whenever there was a problem with the main link or main domain. I think I recall some people mentioning on Telegram they could not access, but they could do through the mirror link.
Also, I am not sure to what extent some webpages are able to tell whether some users are using a VPN to access to their services, I have personally used some software before to hide my IP, (Tor for example) and while using Google, YouTube and Gmail I get messages from them trying to convince me not to use those softwares, because they alledgely respect ones privacy.