Vinegar? How does that help prevent fading? I like to keep my denim dark. Will pick up a couple of bottles tomorrow.
The vinegar bath is supposed to "set" the indigo dye on newly dyed cloth--indigo is not soluble in pure water and the vinegar is supposed to provide a final set to ensure that all absorbable indigo has been pushed into the fibers. I don't know if it is applicable to items that have already been worn, or which come prewashed. I've done it in the past with my Japanese-made indigo-dyed hakama (traditional Japanese pleated trousers, for those who don't know), and can't say whether it really works. Ask anyone who practices kendo, iaido, or another hakama-wearing martial art what to do with a brand-new hakama and they'll probably tell you to soak it in vinegar to set the dye--maybe an old wives' tale, maybe not. Maybe not even applicable if the cloth isn't dyed using indigo pigments.