I was able to figure this one out by trial and error. Dropping the 1st Action element altogether (which called the MachineId parameter) resolved this issue entirely!! Merely targeting the properties of the vCAC Entity was all that was required. Can't explain this but I suspect, given the State of the Machine (Provisioned), that this particular Input is simply redundant and not required or is getting clobbered because of a conflict with another Stub WF in the machine's provisioning cycle invoking this same call??

Nonetheless, happy to report this now works. I'm able to add the VM Owner/Requester to the VM's Local Admin Group without the use of the Windows Guest Agent. I've taken the liberty of attaching the complete package (Workflow and PS Script) once again.

Ron