After maybe 8 months of NOT being able to play my QT files I had rendered in Vegas, I just noticed the A/V controls on the QT Player Toolbar - and played with them. What I WAS getting was a BLANK white screen when the file played - could hear the audio stream, but no visuals.

Well, all I needed to do was to adjust the brightness to any point on either side of the MID setting for the brightness. And now QT works.

I hope this is of help to others.