I have used a lot of different things and have finally just settled on Ducky cleaner. Using a good brush or magic eraser with Ducky cleaner works as good as everything else that I have tried. (this is not the ducky waterspot cleaner).
For grease, there may be something better, but for general boat grime, I am sticking with that.
Honda Cleaner and Polish!! Best stuff out there, all in one, cleans and protects. It works great on the outside as well. You can get it at any place that sells dirt bikes, atv's and street bikes.
DON'T USE Magic Eraser!! It cleans very well, but it can damage the protective coating on good vinyl, making it dull and prone to staining and mildew. I speak from experience.
Along with cleaning as best you can just leave her out in the sun. I left a wet black mirror cover on a seat once that leached the coloring into the vinyl and no amount of cleaning would take it out. I left it out in the sun and within a day it was 90% gone and now has been completely removed thanks to UVB and A.
RPrenner Do the spots look like this photo, If so good luck, nothing will take these stains out, there pollen stains, they will someday fade out, It sucks.
Babe's Seat Soap followed by Babe's Seat Saver. Works great for the day to day stuff. Should get the grease off - may need to use a plastic brush. The Seat Saver keeps the vinyl very nice without the slippery greasy feel of some other protectants.