Even though vinyl, I use (a very sparing amount of) mink oil. Don't use a rag, just your finger tips, and rub it in. You can use a soft toothbrush on the seams. Let dry, and buff with a boot/shoe brush.
Ultimate says their seats are not waterproof (and sells water bonnets for them; I never bought any), and I don't let mine sit out in the rain all day, but still I've never had any of my Ultimate seats retain any water.
You can use leather conditioners (creme or wipes) of various kinds too, just use as little as possible.
Do
not use Armorall, especially leaving it out in the sun. That stuff breaks down vinyl, and makes your seat a greasy slip-and-slide.
