I am sorry but I can not safely advise you as I really do not even understand why your vet would recommend this as a treatment. Can you possibly explain what is wrong or what they are hoping the vinegar will cure? More information about your birds issues might be helpful so I know who I can tag to help you better.
I do use ACV on my sons dreadlocks to rectify the ph balance after doing a cleanse with lemon, salt and bi carb. When I use ACV on my son it is always well rinsed out and the only reason I use it is to restore the correct balance so the bicarb does not eat his hair away. I am not really sure I would be comfortable using it on my bird though.
He has skin condition. It started as a molt. But a few weeks into it, I realized it wasn't all molting and he was barbering as well. Since his food is great and his living condition is a very enjoyful condition, and also that he is 9 months old, we took him to the vet.
The vet checked and said he had so much inflammation in his skin. He asked about his condition, did a full blood work (came out surprisingly super good and normal), then a bacterial swab and then a skin autopsy. He has a skin infection, but none of the tests could reveal why his skin is so inflamed and red and does not get better with medication. Another vet also checked him and the results of the tests and could not figure out what it was.
He has been on antibiotics for 10 days and on an anti inflammatory for about 5 days. The antibiotics and others did not seem to work. The anti inflammatory worked better but that's for obvious reasons, and I cannot continue feeding him medication.
So a third vet gave us a hypothesis. He says that my bird has an unusual skin condition. My bird has an unusual way of feather growing, and they are painful when they want to grow first (which explains why his barbering started with his molt). So the vet's theory was that the bird picks on the feathers and skin to make it less painful for the feathers to grow. But in turn he hurts the skin and it attracts bacteria and makes the skin inflamed and red. He said this whole thing is a hypothesis though. He suggested that it is a genetic issue and it cannot be fixed. He will continue doing that for probably his whole life.
So his suggestion was that we should find a way to make his body less hospitable for bacteria so his skin does not grow bacteria on it as much. His suggestion was that we wash him on a daily basis (for now and gradually make it less often) with diluted ACV and spray it under his feathers and on his skin. And also to add ACV to his water. This would kill the germs that already exist and would make it less hospitable for them to grow again as well. So I forgot to ask him about the ratio of ACV and water and whether I should rinse it off.
Today, I did make a dilute of 2 litre water and 1/4 cup of ACV and sprayed under all his feathers and on the skin, kept it for about 1 min and rinsed it. Surprisingly it seems like it worked. His feet is much less inflamed and red now.
I read somewhere that we could leave the dilute on his feathers and didn't need a rinse. That's much better because that way, I could add it to his bath water and it would stay on his skin to ensure he has that protection. But that should probably have less ACV in water. So I am looking to see what would be the best way to go ahead with this.