We haven't slept on the new seats yet but they were covered in vinyl before and to be honest, we use two sleeping bags, one on the bottom and one over the top, or we would use a sheet and duvet, and I didn't notice any problems. Yes vinyl may be sweatier than fabric but we have two small dogs and I need something that wipes clean and doesn't stain. Also I am aiming to get a memory foam mattress topper - I know it means extra packing but even an inch thick one would make a difference to sleeping for me (I have memory foam bed mattress at home) and then I will make a fitted sheet cover thing that will go over the topper, Velcro shut at the bottom so it can just be squashed up to put away. I'm also getting vinyl front seat covers made, again so I can wipe them clean. If it ever does get hot and sunny, I'll put a cotton seat pad on them to stop the 'sweat/stick' syndrome.
Your quote for professional fluted and piped seats including foam sounds good, the very rough quotes I found were twice that! I enjoy making things, it's nowhere near professional quality but it'll do for us. If it does get too bloomin hot and sticky, I'll make some cheapo cotton covers for the seats
It would be nice to think that this year it will be hot, sunny, dry and lovely for weeks on end, then I'll know for sure if vinyl was a good choice, until then, it's fine, we like it and its practical too.