This article originally started off life as a record of how I managed to get this working, as a lot of my posts do, but this time it appears I am foiled.
Last week, I had 3 vCenter Servers that appeared to be happily talking to each other in Linked Mode sharing a singe Multi-site SSO domain without any real issues. I had a single-pane-of-glass view of all 3 and I could manage them all from the one client.