I think hope this docu is to old to while other things are possible with ipv6



While it is possible to have for example openstack instances with ipv6 static external , then DHCP to for example a eth2 with that ipv6 block detected in virtualmin.
Where you can set the ipv6 to virtualservers as You want… on eth0 the ipv4 a eth1 where another ipv4 … is with routing table…

BUT NO DOCU / help files for virtualmin for those? only

In order to use IPv6 with Virtualmin, your system should have a static v6 address on its primary network interface (typically eth0).

We have Both variants running with CENTOS 7.7 ( the openstack instance is test, the other a small production / test as the manual-help from virtualmin on primary interface)

YOU HAVE TO TAKE CARE of CSF firewall for DHCP client ipv6 on the ports!