Thanks for your help, Shawn.
Regarding the file format, the manual says “Sensor resolution 640×480 VGA CMOS sensor”. That’s the closest thing I could find to a file format description.
I did go to Project Properties, where the upper right tab is called “Match Media Settings” in my version of Vegas. I selected one of the clips in question but that did not change any of the default settings at all. (Which default is set to NTSC DV.)
One other clue is that, whenever I import any of the clips into Vegas, an AVI.sfk file is created. But that doesn’t change being unable to see the video in Vegas.