Hello,
I’ve very new to Vegas and had the same problem with JPG’s being stretched to fill the screen. The only way I could fix it was to right click each image in the media store area and then select properties.
The properties menu then had an “aspect” drop down option that I changed to 1:1. It worked a treat, but I do have to carry out this operation on every single image I import. 8o(
This may sort the issue for video files, but I have not tried.
Best Regards, Alex.