After purchasing a 1992 Hobie 18 with a white vinyl tramp, which was discolored to the point that it looked beige, I went on a mission to find the right cleaning solution to lift the nasty stains.
I started with powdered Comet w/ bleach, and added some pure strength bleach. I pre scrubbed a small area, and then added some CLR cleaner (Calcium, Lime, and Rust). I scrubbed this in and WOW what a difference. I'm pretty sure it caused a chemical reaction, cause it started to burn my eyes, and I actually had to put on a paint respirator. You'll need to do this in a well ventilated area. The tramp looked like new when rinsed and dried. Good Luck and be careful.