Sounds like you need to develop a custom panel. With for instance a dropdown list to select the vCenter and then an AO workflow behind it that reads the clusternames in that vCenter. You will need a quite recent version of CLM in order to create custom panels.
You can find more information about custom panels on the documentation website.
Hope this helps.
Adding to this if you are looking to do a placement on the custom panel values then it is not possible. Placement can be driven using the option choices and tagging logic only.
There is no capability to have options within options in option choice.