Vinegar does not necessarily set tie dye colors, but it does help them to stay especially vibrant, particularly on certain fabrics. Adding 1 cup of white vinegar to a dye lot containing nylon, wool or silk makes the colors more intense.
If the white vinegar is added five minutes after the fabric begins dyeing, the colors are likely to be more level and even. White vinegar can also be used to keep tie dye colors from becoming washed out. After a new tie-dyed garment has been washed for the first time, then rinsed and dried, soak it in a sink filled with warm water and 1 cup of white vinegar for five minutes. Wash it one more time in cold water, and use cold water for all subsequent washings to keep colors vibrant.