[Randy Kahn]

I had the same issue (or similar)..
When I drag or include a pre-rendered video, and then change aspect ratio of this imported clip (by Sony Vegas of course), the change does not get reflected in the new rendered video (with the old one that I imported and the existing one in the new project)..
The Old video rendered as if I did not make any changes in the Pan & Crop to the pre-rendered clip I brought in (to fill out the screen), it was squeezed in.
I was able to fix it by checking the box in the “Render As” dialog box, at the bottom, check th checkbox to “Stretch video to output frame size (Do not letterbox)” in my situation. Fixed it !