Using the vSphere REST API, I would like to make a copy of a vmdk into a new file so that I can create a new disk with this copied vmdk as it's backing. How could this be achieved with the vSphere REST API that was introduced in 6.5?
Unfortunately, that functionality is not yet available in the vSphere Automation REST APIs.
Currently you can only list the VM's disks, get information about a specific disk for a VM, reconfigure which VMDK a VM is using, add a new VMDK, and delete a VMDK.