I agree that surfactants are what would be missing using just vinegar. Surfactants essentially break up the surface tension of water and allow greases/oils to be washed away by allowing them to essentially be dissolved in water. There's oils in poop so surfactants are what allow that to be truly/completly washed away. Most homemade household cleaners use vinegar as an additive don't they? I know that you can do windows/mirrors with just vinegar and water (that's what I use) and that some use it on floors as well but I don't know that many people or recipes use it totally on it's own.

You could use vinegar in every wash and/or rinse to help keep them build-up free (the vinegar will help the detergent rinse out better) but I wouldn't replace detergent and/or soap nuts with only vinegar.

Karen.