This is the information I received when I bought Retayne and Synthrapol. Synthrapol is used to suspend and carry excess dye from cotton fiber. It removed any unfixed or unreacted dye to keep it from redepositing dye onto areas of the fabric that you don't want to be stained. It is recommended for use as a pre wash to remove excess dye (especially in reds and blues), sizing, dirt and oils.
Please note" It only works on fibre reactives dyes, which is generally what is used in quilting fabric. If you have clothing that has run,i t may be a direct dye which will not respond to Synthrapol.
Retayne is used to set dye. For example, if you printed a text message on fabric, you can use retayne to set it. If you have a piece of fabric you know will fade, you can use retayne to keep fading to a minimum. What about vinegar? Vinegar (and other acids) used to be used to fix the dye on protein based fibers such as wool. It was never effective on cotton, although it "does" remove soap scum and other residue, thus giving the illusion of brightening the fabric. Vinegar will also remove some stains (soak full strength) and odors.
This paper also had the following "We were delighted to receive this note from a customer in Australia: I'm thrilled to be able to tell you that the synthrapol which I ordered arrived today and SAVED MY QUILT! The dues which had run all washed out and the quilt is like new. I cannot recommend it highly enough and only wish it was available in Australia where no one has heard of it."
Hope this answers your questions about the products. I always prewash with Synthrapol.