I do not believe IPv6 is officially supported by vCloud Director currently. It appears to be using eclipse equinox to back vCD - so you might be able to get the cell to respond to IPv6 and then do some configuration to the equinox environment, but I am not sure and that seems a little hacky.
Another option (maybe a better option) - would be to setup a load balancer that will respond to IPv6, but forward to IPv4. This could run on a different VM or I guess on the same host running vCD. This should work fine for the GUI, btu I am not sure how the web console would handle this.