I recently installed a third party appliance within vCenter, upon removal the installed plug-in was still present in vCenter and no uninstall package was available.
The only option was then to connect to the Managed Object Browser for that vCenter instance, and remove the plug-in.
1) Browse to ‘https://<vcenter server or IP>/mob’.
2) Select the ‘Content’ hyperlink.
3) Select the ‘ExtensionManager’ hyperlink.
4) Select the plug-in hyperlink from the ‘ExtensionList’
5) Copy the ‘key name’ string and browse back to the ‘ExtensionManager’ page
6) Select ‘UnregisterExtension’ hyperlink.
7) Paste the ‘key name’ string from Step 5 into the ‘value’ text box for the extension key.
8) Select ‘Invoke Method’
9) Refresh the ‘ExtensionManager’ web page, the plug-in should now have been removed, log in to vCenter and the plug-on should also have been removed from the list of installed/available plug-ins.