Once a casino promote themselves to be VPN friendly, there is no need to worry about using VPN. If they blame us or make a story up later due to the use of VPN while in fact we do not do something against the other terms, the casino should be responsible for it. Coming to mirror link, yes it is useful for those who have an issue with ISP but it may not work forever because once the ISP aware about the mirror links, it will all be blocked so players will need to wait for the casino to create new mirror links.
VPN usage is usually a tricky affair which involves a lot of variables which is why it's best to do proper research before deciding whether to use them or not. Same logic applies to mirror links too.
Generally, it's always best to consult with the site's customer care and maybe even save their responses before deciding to use a VPN or mirror link in my experience.