This thread is a few weeks old now but for posterity and searchability’s sake for future users I’ll note that since V13’s launch I’ve had the same issues as Ken, with black frames being inserted in renders. Sometimes it happens on still photos I’m doing zooms on but only if the zooms rotate at all, sometimes on dslr footage the shooter didn’t have level so I had to put a crop and rotation on the frame even without movement. One time it did it on green screen footage but the black frame only covered the background image that was chroma keyed underneath and not the subject on top. If you get rid of whatever is causing it in one spot and rerender, it’ll just do it in another spot.
It’s weird and disappointing and a complete waste of money (especially since I had completely different issues with Vegas 12 on a different computer,) but on the 3 computers I use none of them have any problems with Vegas 11 so I’ll keep using and enjoying that for another year.