For mold and mildew stains, we have two products. First, try Moldaway This is a solution designed to pull surface mold off of all kinds of surfaces. If that doesn't work, use Serious Marine Cleaner. It works great on tough stains. Finally, once you do get the spots off your seats, try protecting them with some Serious Shine.
Without getting to technical, your vinyl comes from the factory with a protective coating that breaks down over time. Once it's gone, keeping your seats clean becomes very tough. Serious Shine provides a good barrier between the vinyl and the elements and may keep your spots from reforming.