Try this.

See if you can shave/grate a little of the soap (about 1/2 teaspoon or less) into some warm distilled water (about 1/4 cup) and make a liquid "shampoo." Stir/beat it with your comb - it will get foamy. Let it dissolve thoroughly before you apply it to your wet hair. Using just a little of the soap/shampoo mix, massage into your hair and scalp. Rinse with warm water. Then, rinse/massage with 1 tablespoon of your vinegar stirred in 1 quart of warm distilled water. Rinse again with tepid water. Dry.

The distilled water does not have minerals and chemicals that will react with the soap or vinegar.

Tell me how that works.

You may find the following link an interesting read.

http://100777.com/health/shampoo