Is your bag a "bag" or some other form of packaging? I only ask because those are unusual dimensions. At least in my own experience, when I've checked cardboard boxes or other types of self-made packaging, the check-in agent usually asks me what's in the bag?

So I could see unusual packaging being more likely to bring out the tape measure, although I agree it seems unlikely to be a problem over 3 inches. And a non-issue if you can get the VS agent to tag it for through-check.