If you want to do IPv6, you pretty much have to do it on an External Direct connect network.
I'm pretty sure vCD 5.5 (and maybe 5.1) will support doing customization of IPv6 via a Static Manual and DHCP addressing. Everything other than the IP is picked up via DHCP check, then set as manual after the fact with the IP set in the VM Properties page.
If you want to do routing of IPv6, as cfor said, you an use VXLAN or even VLAN backed network Pools ... but you can't use the Edge Device (from what I recall). You could make your own router and have it be the hop between private and non-private networks.