I just had this problem with Sony Vegas Movie Studio HD Platinum 10.
I have created a few videos, but this one is longer with more tracks.
So, there was a particular section of video that looked fine on the time line, but during preview everything was completely black, though the audio worked fine. I messed around with it and closed and opened Vegas. I tried rendering the track and got back about half of it, still with blank black areas. After it happened on another section of the video, I thought to do a replace in the Project Media tab of the source video file (and accidentally replaced it with the wrong file), but when I replaced it back with the same original file, then everything was fine. It’s like Vegas just forgot how to get to the file for some frames.
Incidentally, Vegas died shortly thereafter. I think something is overloading my system. I wonder if Vegas has a leak somewhere.