Flicker in video when editing/rendering in vegas

This is a bug in Vegas 11 that (now with the announcement of Vegas 12) will not be fixed. Resizing with Pan/Crop or Track Motion can sometimes cause this flicker. Moving an event a few frames either way on the timeline sometimes fixes this but most often it does not. If you are interested in why it happens that is complicated. The short answer is that the Vegas 11 bugs are mostly based on the different aspects of the program suddenly exchanging bad information. It has a lot to do with the mathematical calculations in the different color spaces being given incomplete information. This causes the flicker as well as errant single color frames (from strange i/b frame data ... i.e. your idea of bad codec reading.)

I have had the flickering make it impossible to keep an event they way I wanted a few times. Usually though I can find a suitable plugin chain that somehow side step the mathematical error if I can't change the crop size in any way.

The mathematical errors that make these odd things happen "appear" to be fairly random. As you have noticed, though, changing things around can make these errors disappear. In my experience the fixes remain consistent. The errors do not come back on the same event once you get rid of them. You can never predict when they will show up in another event, however.