Are you having problems with the cushions or the actual plastic portion?
I cam across a product this year that I've fallen in love with, some might not agree but its my go to for bring back my vinyl cushions and tough spots on my 21 se, especially the bilge.
Its called x17, it does contain some bleach. Whenever I get spotting on my captains chairs I spray them down, let sit for a few minutes, and rinse. Also any mold or mildew in your bilge or battery compartments I also spray with x14 than rinse and it melts away the build up. Its literally turned into my go to for getting rid of many stains and mold/mildew just melts away. I haven't had any issue with it causing any harm to my boat or any boats vinyl, but my clothes don't feel the same if I accidentally get it on them. (I work at a dealer ship and use it on new boats all the time without hesitation). But as always, test a small spot first