I did what you said and "quit" VCD assistant. Nothing happened. I then Force Quit it and nothing happened. But I did get a weird message: Do you really want to quit “VDCAssistant”?The process “VDCAssistant” is owned by _cmiodalassistants. I restarted but it made no difference. I even went to my test account and still no camera access.
When you hit quit/force quit, the process should have disappeared and reappeared. If that didn't happen after you responded to the message with a "yes" or whatever the appropriate positive is, you'll have to go the Terminal route.
Copy & paste the VDC command, hit "return," and I again suggest a restart. (You'll be asked to enter your password, but you won't see any indication that it's been entered.)
The bit of searching I did suggests that quitting the process may be your answer.
