I appreciate the information, Francois. Below is the MediaInfo text you requested. Although I deleted the (disappearing) video file from my project and my drive, then went back and restored it from my trash in order to run it through MediaInfo so you could look at it. Hope that didn’t change anything.
In the meantime, I had tried downloading the video clip again from its source and re-importing it into Vegas. It worked for a while, now the whole project cannot be opened. I’ll have to rebuild the project from an older backup.
An odd point that may or may not add insight: I pulled up an old version of my project and the video files that disappeared in the more recent version of my project — those same files play fine in the old version.
Here’s the information: 
Thanks for getting back to me so quickly!