I just ran into the following issue when I upgraded the customers vCenter Server to 2.5 U4: “”Failed to install the VC agent service” Error Message Appears after VirtualCenter Upgrade”. None of the 3.0.x hosts would connect anymore. Of course the knowledge base provided a simple solution for this known issue. I thought I would document it as well in case I would run into it again. For me just restarting mgmt-vmware and vmware-vpxa was sufficient.

http://kb.vmware.com/kb/4223326

To resolve this issue:

  1. Disable VMware High Availability (HA). Otherwise, the virtual machines might be forcibly powered down by step 2.
  2. At the service console, issue the command:
    service mgmt-vmware restart
  3. At the service console, runthe command:
    service vmware-vpxa restart
  4. Reconnect the virtual machines to VirtualCenter.Attempt to re-enable VMware High Availability (VMware HA) within VirtualCenter. If this doesn’t work, this means that vpxa did not install properly.
  5. At the service console, run the command:
    rpm -qa | grep vpxa
  6. At the service console, run the “rpm -e” command on the rpm file that displayed in the previous command.
    rpm -e <rpm from step 6>
  7. This uninstalls the agent from the ESX host. Reconnecting the host to VirtualCenter re-installs the agent.
  8. Reconnect the virtual machines in the usual manner within VirtualCenter.
  9. Re-enable VMware HA.