My boat had discoloration on the small panels that had the embossed logos at the tops of the front and back seats. The vinyl was pretty glossy after 20 years of use but it was in great shape. Using a grey scotch pad (a few of them) I scuffed the seats and the engine cover after repairing a few nicks in the vinyl. I cleaned the areas using prep-sol and dusted on about 3 medium coats. I dries as a very matte finish but a few trips out tones it down. My after pictures have about 8 ski trips on them. It literally makes the seats look like new. I used Formula Boats Grey which is about as close as I will get to original. It is extremely close to the color, maybe a little more grey which I think looks better. I'm going to do the side panels this fall. I think SEM only has about 15 or 20 colors to choose from. This is marine vinyl dye which is supposed to have more UV protection. One word of caution, do not get overspray anywhere you don't want it, especially on other vinyl panels of a different color!
The first two pics were when I first looked at the boat at the previous owner's house before I bought it last fall. The last two are from today.