I used to just use marine soap on the seats. This year, I used a degreaser diluted fairly weak and followed up with Hammerhead Boatworks'
Vinyl Protectant.
I don't think you'll go wrong using auto shampoo, but I don't know if you'll get the cleaning action you want (especially with boat seats).
PS: The vinyl protectant smelled awesome and left the seats dry to the touch. I'll post some pics when I'm not as lazy and the boat is finished.