VMware Virtual Machine collections is not a standard feature for custom reports. VMware Cartridge Out of the box reports do limit themselves to the Virtual Machine objects within a Service.
There is an alternative for custom reports, which is to create a Custom UI Query.
To create a Custom UI query for VMware Virtual Machine objects.
Note: The Max Search Depth parameter can be changed to 1 or 2 if the service is updated by a rule. More than 2 is not recommended as it might put unnecessary extra strain on the FMS server.
If done correctly, the Service should now have the entry below that can be used to create reports for VMWVirtualMachine objects.