Its mostly an old wives tale that vinegar will set the fabric colors.In fact, vinegar can do nothing useful for cotton dyes. Vinegar will help set some acid dyes, but only if applied while it is gradually heated to a simmer (generally in the presence of salt), solely in cases in which this necessary part of acid dyeing was omitted; acid dyes are used on silk, wool, or nylon, but never cotton.
If you want results use Retayne prior to construction on fabrics, and use Synthropol in the wash of finished quilts. Color catchers work well with finshed quilts as well.
True Dye Fixatives, an all-purpose solution
There is only one type of product that you can buy that will actually set dye regardless of its type. A product called Retayne, sold by local quilter's supply shops as well as by most mail-order dye supply houses
Retayne and other commercial dye fixatives are the only real solution to cloth that bleeds.
is recommended for fixing dyes in commercially purchased cotton fabrics or clothing to prevent color bleeding during washing. This product is a cationic bulking agent, which acts to seal in the dye by physical means, rather than the chemical bonds which are so dependant on the type of dye. It seems that the particles of Retayne adhere to the dye molecules, effectively making them larger, so they do not come out of the fabric as easily. Note that Retayne is washed in as a laundry additive, and thus can be used only on things that can be immersed at least once without the dye immediately floating off and ruining other parts of the same item. Retayne may be removed by washing with overly hot water, and thus treated items must be washed in cool water. (Unfortunately, the one situation Retayne cannot help with is the dry crocking of indigo, in which improperly applied dye rubs off of the fabric even when dry.)
Instead of Retayne, G&S Dye sells a similar product called Raycafix, which they say is stronger; another advantage of Raycafix is that it can be laundered in warm water without losing strength. Dharma Trading Company sells Retayne and also a product called "Dharma Dye Fixative", which they claim increases the washfastness, wetfastness, resistance to perspiration staining, and resistance to seawater fading of several different kinds of dye. I don't know how it compares to Retayne; it may be a generic version of the same product, or a related product, instead. Aljo Dyes strongly suggests the use of their equivalent product, Aljo Pro-fix PCD after-treatment, after using direct dyes, which would otherwise run too much to be at all desirable. Note that the use of any such aftertreatment, while improving washfastness, may adversely affect lightfastness.