To the best of my knowledge it appears that when objects are removed from vSphere, they are remove from vCops. You can still see them in the Custom UI with a blue question mark, but all of the metrics are gone.
For example I rebuilt my cluster replacing AMD hosts with Intel hosts. Since the servers are new objects, I have no way of comparing the metrics to understand performance gains for the Intel servers as the AMD data is gone.
Thanks,
-MattG