Vinegar has no effect on today's modern chemical based dyes. If your red fabric that you are using in the new project is still in yardage you may want to try washing it in a dye fixative like Retayne to prevent fading. But fading like you describe sounds like a problem that nothing will fix. I would ditch the fabric and try something else.
This web page has a lot of useful info but is geared mostly towards denim but explains a lot:
https://dengarden.com/cleaning/How-t...ing-and-Fading