If it's not caused by FAT32 file system size limits or a bad encoded source video, it might be an issue about Vegas engine saturation.

Vegas render engine gets saturated when there are clips with large frame sizes with too much effects on timeline.

It's very easy to experiment it.
1 Just open a new project.
2 Insert a video track and set it's composition mode to 3D Alpha.
3 Get a still image and put it on the track. (or pick a texture from Media Generators)
4 Enter Track Motion and rotate the frame in any direction.
5 Set the preview screen mode to Good (Auto).

6 Press Play on timeline... It should play nice.

7 Duplicate the track and play it again.
8 Repeat step 7 until your Vegas gets saturated.

While rendering a frame, Vegas ignores cancel commands. It waits the render engine to finish the frame and it executes your abort command while moving to the next frame.

On my PC, I can have 8 tracks maximum. More than 8 gets Vegas into [Not Responding] status.