I have to add my two cents worth from a scientific/veterinary perspective
The proventriculus has a pH of around 2.5-3, trust me, it does not need vinegar to improve the acidity. As for bacteria, well, unless they are encapsulated in something that bypasses the proventriculus (and some do make it hidden in food particles etc.) then they are going to be annihilated by the stomach. It's one of the bodies defence mechanisms to try prevent nasty bugs entering our body via the oral route.
The vinegar may prevent overgrowth of some bugs and yeast in water so its fine to use but it shouldn't be used as sole treatment of a bird that has been diagnosed with candida etc. Candida can grow out of control super quick and become systemic and kill the bird. To me its not worth the risk when we have brilliant veterinary drugs that can treat the disease.
However, I am not completely opposed to natural remedies and I think using vinegar in water is fine as long the limitations are recognised
