I’ve been struggling with Sony Vegas for about two years when it stopped opening avi encoded with different codes, and also mp3, mov and most of mp4.
I recorded some desktop video with Camstudio, which failed to open in Sony Vegas, even though VLC can play it. None of the files dropped on Vegas Pro could be opened.
If you need to stretch a video piece of another aspect ratio to fit the screen in Sony Vegas, just right click on it, select Properties and deselect Maintain aspect ratio.
If you upgraded Sony Vegas you’re probably now struggling to open projects saved in previous version. The problem is when you try to open older projects in a newer version, Vegas merges all media files inside the project in a monolithic piece, which practically makes editing such project impossible.