Hi,
The vdc.createDisk() method returns a disk object with a running task in it. It doesn't wait for the task to finish (this is true for all other similar methods).
You can wait for the task to finish by calling: disk.getTasks().get(0).waitForTask(0);
The current version of vCD doesn't allow saving disks as snapshots or cloning disks. Probably you can achieve such functionality by using the vCenter API directly.
Regards,
Todor Todorov