Didn't know about Onyx so I spent some time trying to get that working. Unfortunately it breaks vCenter's web client. Supposedly the requirement is vSphere Web Client Version 6.0.0 Build 2559277. But I can only download later versions from the VMware site. (Even 6.0.0b is a higher version than what is required.)
Anyone have a working installation of Onyx on vCenter 6 and could capture the PowerCLI for assigning a vCenter 6 license to an asset?