Chemistry - Will using vinegar harm car paint?
That is a valid question, because acid rain has been known to damage paints, particularly high-gloss, i.e. clear coat. Best would be to test the effect of vinegar on a hidden area, and double-check under bright light from different angles. Your idea to test on glass is good, because it shows how long a soak is needed.
If it appears unaffected, rinse afterwards with a weak solution of baking soda or ammonia, and then rinse again with plain water to remove any remaining acid.
If damage is slight, can car wax effect a good repair?
If vinegar does damage in the test spot, you might look at various car polishes or even cutting compounds, and then wax (or reapply a gloss coat, which may mean refinishing a large section of bodywork to get it to match).
Vinegar Test: No Visible Change After Two Hours
As advised above, I decided to test vinegar on an inconspicuous spot.
First I created a pattern of exposed paint using masking tape. The shape was just something random that I thought I could most easily recognize, even if just a slight difference in the car's finish:
Then I placed vinegar-soaked cloth on the test area of the car:
After two hours (which was longer than I expected the cleaning of the other parts of the car to take), I removed the cloth, rinsed the area, and finally just washed the are normally:
And finally took the car into various lighting conditions. There was no detectable difference in the finish that I could see: