I feel your pain, had the same stuff in the Birmingham area. I first noticed it on my YJ which already had some stains on it, the red clay was a more even stain that didn't look as bad as the initial stains
Here in Mobile there is some red clay but its mostly just dark dirt.
I never let it sit on the top of my TJ, preventing the stains entirely. I know with clothing you could use some borax with water and let it sit for a while before rinsing with cold water. I've never tried it on the vinyl so I would spot test beforehand (if nobody comes by with better advice).