Is there ANYTHING running in the old vCenters at all? If not, you should have a standardized decom process similar to this:
1. Backup Server & Disable monitoring
2. Shutdown Server (wait 7 days)
3. If no issues after 7 days, power ON VM & remove from AD (Windows Servers)
4. Power Off Server
5. Ensure DNS entries have been removed
6. Remove from IPAM, CMDB, CI, whatever you have for that
7. Remove monitoring