I'm clueless with this problem. I had this problem in VP15 and now it's in VP16.
The fact is that in the video with the H264 codec, black sections begin to appear gradually. These black sections are then in rendering or rendering crash. They appear as a disease in more places, more and more.. All you can do is restart Vegas. But this problem will probably appear again after a while.
It's very random and I can't identify the cause of the problem.
Have you ever encountered this problem?