It Sounds Dumb...
...but I always start with a shot of WD-40 on any stain- if it doesn't work, then go on to the stuff noted by the previous posts. I've also had reasonable luck on stubborn stains with the simple eraser on a number 2 pencil. If all else fails, try Comet Cleanser, but this may abrade the vinyl a bit. I have a few tan stains on my forward seat from the boat cover I use that uses the seat for some of it's support. I have found nothing that will touch these stains since the dye from the cover appears to have soaked into the vinyl.