You have told that they said using vpn is okay. In this case would you like to share the link/screenshot of that where they have said okay regarding using vpn. If they said it is okay then there is no reason to lock your fund and if still they lock then they are nothing but a scammer.
After having read a few TOS agreements recently, I can tall you the following thing. If a TOS agreement says one thing, but the site's representative says the opposite, the rules in the TOS prevail. We might not like it, it might be morally wrong and even scammy, but the TOS is what you agreed to and not someone's opinion about it on a forum.
In other words, the SandBoxCasino forum admin can say: sure, use VPN. We don't mind.
But if you do and the casino locks your account, they can sanction you based on the agreement you gave your consent to. Many sites even have clauses in TOS agreements where it says that in case of conflicting information between a TOS and any other public information, the TOS prevails.