DPM cannot backup a VM in snapshot/online mode if that VM has a volume that has VSS enabled and it set to store those snapshots on a different volume.
1. Is this a limitation of DPM or the HyperV VSS writers?
2. Can other backup products that use the HyperV VSS writers backup the VM while it's online?
3. If a virtualized domain controller is being backed up by DPM as a VM, is Active Directory being backed up properly? If the Virtual DC is begin backed up with saved state mode, is AD backed up properly?
http://technet.microsoft.com/en-us/library/hh757970.aspx