The quick answer is VCD does not support IPv6; that said it really only has issue (when talking about vApp networks) at the configuration side of the application.
If you want to use IPv6 set the gateway to a valid IPv4 gateway (the edge device will only use IPv4) and then once the vApp is online with IPv4, go to each VM and add IPv6 entries a you require. Your VM's (if using vxLan) will be able to talk via IPv6 (however the gateway out of the network and connections in will need to be IPv4).