I don't see how vinegar in the humidity tray would affect the pH of your growing media unless the pot was in contact with the water. In that case, water would wick up into the pot, likely creating a perched water table. It would be better to add the vinegar to the water you use to water the plant. The usual advice is to test the pH of your water and then add enough vinegar to bring the pH down to about 6. I know my water has a pH above 8, so I add a Tablespoon of vinegar per gallon to my water when I water my plants.