Microsoft Windows 7 now supports the use of Virtual Hard Disks (VHDs). Microsoft does not support backing up a physical disk and a VHD on that physical disk in the same backup job. This limitation also applies to Norton Ghost. You cannot back up a physical disk and its VHD counterpart in the same backup job using Norton Ghost. Also not supported is the ability to back up a VHD that is hosted on or “nested” within another VHD. If you want to back up a physical disk and a VHD on that disk, you must create separate backup jobs for each disk.
Backing up a physical disk that hosts a VHD is supported as long as you do not include the VHD as another volume in the same backup. If you backup a physical disk that hosts a VHD, the VHD is treated as another file that is part of the physical disk backup.
VHDs can be attached and detached from their physical disk hosts (volumes). Microsoft recommends that you detach a VHD that is stored on a host volume before you back up. Not detaching a VHD before you back up a host volume can result in an inconsistent copy of the VHD in the backup. After you restore a host volume, you can re-attach the VHD file.
http://entsupport.symantec.com/umi/V-306-2
You can find more information on backing up VHDs on the Microsoft Web site.
http://technet.microsoft.com/en-us/library/dd440865(WS.10).aspx