What happened was that I loaded up a customer scene, activated the vertex paint brush, and it didn’t work. But I had just been painting vertex colors before I loaded their scene! I didn’t think to check the brush properties until later, so I spent a bit of time scratching my head over this…until finally I noticed that Selection was set to Use, not Ignore.
I tested this by saving scenes with different brush settings, and then reloading them. And different brush settings came in with each scene.