Turns out that there were a couple of mis-configurations on my part:
I used a domain service account to register the plug-in, and when I looked more closely at the vCloud Director plugin under "Administer" in VCO, it was registered to "[serviceAccount]@[domain]@system". So...not a good start.
Once we created a Local vCloud Director account and registered the plugin with that, it looked more like "[LocalVCDOrchestratorAccount]@sytem" and the underlying objects appeared as expected in VCO. (When registering, I did not include an "@sytem" in the user name.)
Some other information that we used during registration of the plugin: "Basic Authentication" and "Shared session" (in addition to using "system" as the Organization).
Everything is working as expected again.
Thanks to both of you for the input. Definitely tracking right along with what ultimately solved the issue.