Great blogpost By Robin Hobo explaining how to update an XenApp 6.5vDisk with PVS versioning
Back in the old days when you wanted to update a vDisk, you needed to make a complete copy of the current vDisk, then import it back into the vDisk Store, set it to Private Mode and link it to a Master device to apply your changes. After making all the changes, you needed to put the vDisk back to Standard Image mode and copy the complete vDisk to the rest of the Provisioning Services servers.
Now with Citrix Provisioning Services vDisk versioning you can add versions to your current vDisk. This method of updating the vDisk creates a delta file next to the vDisk itself. This delta file is usually considerable smaller than the complete vDisk file and you don’t need to change the vDisk mode to apply changes to it so that it can continue to be used.