To provision a vmware host via IPv6 only, you have to boot the vm via efi firmware and set the "networkBootProtocol" to "ipv6".
Fog-vsphere currently does not support this setting as rbvmomi only supports api revision 5.5 and the setting needs at least 6.0.

There is a PR open that allows rbvmomi to use api revision 6.0, but the project seems rather unmaintained.
https://github.com/vmware/rbvmomi/pull/77

We have to make a decision if we add a per host parameter which network protocol should be used for provisioning or if we just add that at the compute resource level and offer a host "both" possibilities.

Documentation for the required setting:
http://pubs.vmware.com/vsphere-60/index.jsp?topic=%2Fcom.vmware.wssdk.apiref.doc%2Fvim.vm.BootOptions.html