If this were the REST API, you just GET the VM information then POST it back with the changes as required.
There is also a forum specific to the vCD API:
https://communities.vmware.com/community/vmtn/developer/forums/vcloudapi
I can't seem to find a forum that's specific to the PowerCLI for vCloud.
i think actually you cant change anything on you vcloud vms via the vcloud powercli, you can only start-civm/stop-civm/stop-civmguest/suspend-civm your civm's
example: Get-CIVapp "vappname" | get-civ -name "vmname" | stop-civm