Once per a time it happens that VD hangs up and does not respond. It may take forever so the only way to go is to kill the application.
I think I found one cause that may cause it: VD somehow waits for the information from some application about changing the state, and this call blocks forever. I found this hangup report on Windows 7 and it was stating that the VD is waiting for some application for closing. That application actually hanged up. Killing this application helped and caused that VD was unblocked.
The problem for the user is that normally they have no clue why this application has hanged up and they expect VD to help solving this problem, not just hang up and not respond. Would be good to have some timeout and assigned it some hangup state so that VD can normally function.