The fading/oxidation you're getting may be from the protectant. (If you want to ruin something, use Armor-All) Marine vinyl is made to be fade-proof (or very close to it) & doesn't need UV protection. You shouldn't need to put anything on the seats to protect them from the sun. Covering them will make them last longer, but often that's not practical. Those seats should remain good in the weather, untreated & uncovered everyday, for 10 years or more.
Marine vinyl is some good stuff & I couldn't imagine Bennington cheaping out & using an inferior material which is why I suspect the problem is caused by protectant you used. 303 was designed to be used as an interior, not exterior, protectant on aircraft so perhaps the sunlight is causing it to crystallize. You don't need the stuff.
1994 Tracker Party Cruiser
115 hp Merc, 2 stroke