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).