You would want to be careful with any cleaners with Sodium Hypochlorite, chlorine bleaches, they will degrade the vinyl and can hurt the material they are put together with.
A vapor steamer will work great with this. The problem with Vinyl can be the oils or plasticizers that keep them flexible. I have used the vapor steamers for just such a job and it can do a pretty good job.
Well to clarify I was at a boat shop once and they make a vinly cleaner for mold (don't remeber the name) and it has a lot of bleach in it, it was the only product that would remove the stains from the vinly on my dad's boat. Prior to that we tried general purpose cleaners and peroxide based oxidizing cleaners with little success. The bleach product did not hurt the vinly, I think it should be fine to use on an occasional basis, after cleaning it is also good to follow it up with a good vinly conditioner like Armor All or something similar.
Daron, Steve, Rickie......thanks for your suggestions. I'll try them one by one.
I've had those ugly stains on them seats for a few years. I hate to look at them each time I go out fishing....boats not that old... 2000 Bass Tracker. I took the cover off one day and wal-la, mold stains.