I think the latest vCAT states:
Virtual machines managed by vCloud Director using IPv6 can only communicate to endpoints that are not behind vCloud Network and Security Edge (Edge) devices. Edge does not currently support IPv6. Virtual machines that communicate on the same directly attached vApp or organization virtual datacenter network can use IPv6. To communicate with the outside world using IPv6, connect the organization’s virtual machines to a direct external organization virtual datacenter network.
I do think we state support for IPv6, but I can't locate the restrictions placed on it. If I were to guess, this seems really similar to a post here: VMware vCloud 5.1 Networking for dummies part 2 - Gabes Virtual World
Some of the fields on that configuration page don't have any checking going on, others do. Let alone the masking is more like a CIDR format which would use a forward slash / then the mask (64 in your case).
I just don't recall Static IP Pools being possible with IPv6 addressing ... just Static Manual addresses.