This isn't like the vinyl that was in autos 40 years ago. The new stuff is pretty good and the heat factor is almost a none issue. Will it warm up, sure. Will it burn you? Not that I've seen on the boats that I'm on sitting in the sun all day.I am scared of the heat factor of the vinyl. I am getting cloth and putting neoprene covers on the cloth seats.
As for neoprene, as long as you wash them occasionally. Have you ever been around a sweat soaked wetsuit? Nasty!