I am running orchestrator 6.0. I did not attempt to configure the vCenter PlugIn in the Ochestrator Configuration webpage, as it is no longer available there, but went into the Orchestrator client. Under Design, I went to the "Add a vCenter Server Instance" workflow and ran it, using the same credentials I had just used to log into vCenter Web Client. Those credentials happen to also be the SSO administrator credentials. When I ran the workflow, it appeared to complete OK, but under "Variables" -> "New Instance" it says "invalid credentials".
Any ideas?