Sun block and lotion combined with sweat will cause white vinyl to get a yellowish stain. My wifes bolster turns and mine doesnt because I dont use either.Usually if you wipe it down with bleach and then water right after it will go away. Keep the bleach off the stiching!!

If it is yellow, I would bet it is mold, and not mildew. I have a friend that had this happen to her BMW that she kept in a hot garage in Dallas.

Whatever I had; NOTHING would touch it. I tried everything. Thus I think it was under the vinyl.
Now it looks stunning!

I don't know what the hell it is, but I'm going to try x-14, whesley bleech, and if no luck then good old fashion clorox. In that order. Hopefully, one of them will work!! I will let you guys know.

You might be on to something here, sunblock + sweat = yellowish stains on my t-shirts. I just shoot some Shout or something on it before I throw it in the laundry.

Use the bleach a cup or two in a gallon of water with some laundry detergent. It'll make the skin on your hands peel, so wear gloves. Let it sit for a while then scrub with a brush then rinse. On bad stains I've poured it on full strength. Don't drop it on anything with color like the carpet. My ski boat is 12 years old and the vinyl still looks great.

Might be worth trying those Mr Clean Erasers before tryiong bleach. I used some Wesleys on white vinyl and it seemed to remove the top layer of the finish. It's never looked right since I tried Wesleys on the vinyl and it's never changed anything else. THe erarsers seemed to work very well with out chaning the vinyl.

I second the Eracers. It worked pretty good on my interior

Nothing works. X-14, Whestles, bleach, Nothing will remove these yellow stains. I going to look behind the vinyl and see if its coming from behind. I'll let you know.

Try RollOff and get some light on it.