vinegar does not set the color in quilting cottons-
it will however freshen the water-
vinegar only works with acid dyes----which are used for wools- and other fibers-besides cottons
i could never understand why anyone would wash fabric- quilt or otherwise- without detergent---seems like a waste of time and water to me-
i always use detergent- i want to wash away any sizing/chemicals/oils in the fabrics
i do not use fabric softener when pre-washing fabrics- mainly because it can effect how fusables or other treatments you might use while making your quilt- i do however use fab softener when i wash the completed quilt.
but unless you need to adjust the ph of your water- you are just wasting vinegar.