A few benefits of ACV:
ACV is not particularly nutrient dense, aside from potassium which it contains 11 grams per tablespoon, it really has no other nutritional significance. So added nutrition is not why we use it.
ACV is quite acidic so when using it either internally or externally it should be diluted to be safe.
For a topical use, 1 part ACV to 3 parts water is recommended. This dilution can be used for a number of things: a paw soak for yeasty or irritated paws, a spray solution for hot spots & yeast, or even natural flea/tick repellant, an ear cleaning solution or a post bath rinse to remove any soap residue or odor from your dog. It is even quite effective at removing tear stains, just be sure to keep it out of your pets eyes.
Internally it should also always be diluted in some manor. This can be done with water or broth, or my preferred way of incorporating it into my dogs diet, mixing into food, specifically the vegetable puree's I feed. Now some will say this mimics the stomach contents of a prey animal, well not exactly but it is a good way to get some in the diet safely.
How much do you feed? Well there is no set in stone answer here. One thing is for certain, a little goes a long way & you can definitely over do it. So keep the amount small, say 1/4tsp per 10lbs of dog. Unless a specific medical condition warrants it that should be plenty. I personally add a small amount to large batches of pureed vegetables, about 1/2 -3/4 cup per 16oz of veg.
What you want to look for when buying:
The Bragg's brand is the brand I use & prefer, it meets all the criteria.
!!! Now there are certain situations where you should not use ACV internally with a dog. If your dog suffers with too acidic gut then it is not advised to feed. If your dog has sensitivity to apples, do not use it.
Also, I will say many people recommend adding ACV to your pets water. I do NOT recommend this & do not think this is a good idea. This can detour your pet from drinking their water, not a good idea.
FYI...a solution of 1/4 cup ACV to 1 gallon of water also makes a great pet safe, chemical free floor cleaner!