TO OLD HELP ? DOCU.
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!