If you are using the VMware vCenter Server Apppliance, you must unregister a vCenter Server 5.0 system from the vSphere Web Client using the command-line.

Because the admin-app user interface is not available on the vCenter Server Appliance, to unregister a vCenter Server system with the vSphere Web Client, you must unregister from the command-line using admin-cmd.sh.

Enter the following command on the vCenter Server Appliance:

/usr/lib/vmware-vsphere-client/scripts/admin-cmd.sh unregister

https://<WebClient_IPorHostName>:<WebClient_HttpsPort>/vsphere-client <VC_IP> <VC_Admin-User> <VC_Admin-Passwd>