I mentioned this in another post, and don't have a whole lot to add to what I've said already, but at least this way it is easier to find the info.
The vinyl on our Epics is much more textured than most other boats -- cleaning with a rag doesn't cut it. When I bought my boat, I was all bummed out because of how UV-faded I thought my vinyl looked. It was very much an off-white color. It wasn't until I brought out a brush and some Simple Green to clean off melted nail polish (yes, don't let the ladies sit on their hands on hot vinyl!) that I realized it wasn't UV-faded at all.
I've used three products with this method: Simple Green, 409, and a SeaBolwd (I think that's the brand) vinyl cleaner and polisher.
I highly advise against using Simple Green unless you absolutely need to. The stuff is so potent it will literally eat away at the stitching. I used the Simple Green to clean the nail polish -- but was sure to keep the product away from the stitching.
As for the 409 and the vinyl cleaner/polisher, they both worked pretty well. I'd recommend using the vinyl cleaner/polisher, because 409 is pretty hard on interiors as well, though not as harsh as the Simple Green.
Anyway, I snagged a couple friends after skiing one day, and we went over the whole boat in about an hour, scrubbing the heck out of each of the vinyl panels. There's really not much more to it than that -- scrub one panel, wipe the grime away, move onto the next. I was concerned about using a heavy brush at first, but it didn't do anything bad to the vinyl at all, but gets all the crap out of the deeper grooves in the vinyl. It was also nice to use the brush to get in-between the vinyl piping and the fiberglass (like above the passenger-side seat)...mine were pretty gross.
After I finished going over the whole boat, I went over it again with some 303 UV-protectant. The stuff is expensive but well worth it. You just wipe it on and it keeps the vinyl from sun-fade.
There you go. My back seat is brand new (stitching ripped out, like many others have had problems with), but you can't tell the difference in color between the brand-new vinyl and the old. And my boat's in its 4th summer of use! I was stoked.
I'm going to do this at the beginning of each summer from now on and it should keep it looking new.