
2 tbsp of ACV/quart seems like a LOT. Part of this is based upon the lower water consumption that a chick versus a chicken will have. Just like some vitamins if you read the package put a much higher concentration of the powder in the chick water than in the adult chicken water.... I'm sure that Jeff Smith of cackle hatchery is on to something. Perhaps the chicks will drink more water when vinegar is added. Conventional wisdom I've heard says: "make sure the chicks are drinking water before introducing any feed, as one way to prevent pasty butt." Another excellent product is the green gel called 'gro gel'. putting even a pinch in the brooder water seems to help the chicks. Shippers sometimes make it into a goo and include it in the shipping box.
Right now I'm finishing up some 'rooster booster' brand "Vitamins & electrolytes with lacto bacillus". -- comes in an 8oz plastic container and the directions say add 1/3 tsp per gallon. Some folks are adding save-a-chick -- but that is 3.00 per envelope and 1 envelope goes in a gallon of water as I recall. If you put out 10-15 gallons of water per day you could have a very high electrolyte bill. This stuff - a little goes a long way even though the container isn't cheap.
It's interesting that no one ever makes a consideration of the beginning pH of water in the vinegar discussion.......
If your chickens get water from rain-water, it is more acidic than other water in most places in the USA.
This is from googling--->
What is the pH in rain?
The scale ranges from zero to 14, with pure water at a neutral 7.0. Most water, however, is not exactly pure. Even clean, normal rain has a pH of about 5.6. This is because it reacts with carbon dioxide in the atmosphere and forms mildly acidiccarbonic acid before it becomes rain.
Plants can be very sensitive to pH of soil and water in soil can affect pH. For examples where I live now I can grow blueberries in the ground, where I lived before they died in the ground -- due to soil -- In containers where I lived I watered them with some vinegar in the water and they did o.k.
If your chickens get tap water the pH would in theory be neutral. IF your chickens get rain water -- the pH would be slightly acidic.
Have you ever noticed how when you put out clean water -- and some spills on the dirt---That's the water that the chickens think is really yummy? that's what mine do. Now I think that there are minerals that they get from drinking the dirty water that the waterers lack.
I'm not sure that the idea of putting three bowls of water in front of the chickens would necessarily indicate what is best for the chickens. If I put three bowls of feed in front of my dog - or a cat, horse, cow--- I don't think that they would select the one best for them. I don't think chickens are any different JMO. I could easily be wrong on that. Weather and circumstances could also be a factor. In Gail Damerow's book
Storey's Guide to Raising Chickens - she lists one solution for feather picking for when the chickens have insufficient salt in their diet. Here'sthe quote
"Since salt deficiency causes chickens to crave blood and feathers, try adding one tablespoon of salt per gallon of water in the drinker for one morning, then repeat the salt treatment 3 days later. At all other times provide plenty of fresh, unsalted water"
p. 121
There is a range of tolerance and amounts of acid vs. base pH for chickens. ETA - google says the pH of ocean water is 8.1 or 8.2 for example....compared to the 5.6 of rainwater.
Influencing factors are probably pretty enormous. From my perspective if you provide your chickens with their needs within moderation and common sense - you will be giving them good care.
There is another factor for using vinegar in the chickens waterers, and that is that it helps them with the calcium carbonate in their shells. Vinegar will assist in dissolving the calcium in oyster shells, or the crushed egg shells you feed back to your chickens.
There are even calcium supplements for humans that dissolve egg shells in vinegar.
Here are two articles from a quick internet search:
http://www.fresheggsdaily.com/2014/04/soft-shelled-or-rubber-eggs-causes-and.html
http://fresheggsdaily.com/2014/06/the-danger-of-feeding-too-much-spinach.html
That reminds me that when my first two chickens were beginning to age and lay soft shells, I was able to get them to produce better shells by adding vinegar to their water.
No two people will approach chicken keeping the same way. Everyone's environment, soil, chickens, feed bags, water and supplements are different. You will probably find what is best for you by listening to a variety of view points and then experimenting with what things produce the best results in your flock.

Good luck with those chicks!!