Funny, I was considering writing a research article on this.
There ARE actually studies done that show that acidifiers in poultry drinking water can reduce the overall parasite load in a chicken. It’s pretty standard in large scale meat bird production.
But this is not regular old vinegar bought from the grocery store, and these studies are all done on mass broiler populations, not backyard chicken flocks that we generally expect to live for much longer.
A small amount can help reduce algae growth. Personally, I’ll be sticking to clean, fresh water daily. I don’t have the conditions to be concerned with coccidiosis, and if I see worms in stool then I’ll buy a real dewormer.
So it’s not a total hoax, but it’s not a miracle liquid either.
Everyone has their own anecdotes; I highly recommend doing the research and weighing the pros and cons for your own personal flock and maintenance routine.