there are several manufacturers and grades of marine vinyl in terms of chemical make-up. Avoid anything with a solvent in it as vinyls are a petroleum product. Test an inconspicuous area first. Westleys is effective. SoftScrub leaves too much residue to rinse away. Baking soda is relatively inert and safe. I have always used spray nine with excellent results, no negatives I have ever seen over 15 yrs and six boats. For tougher stuff you have to let it set and soak, or multiple applications. Don't forget...be patient, a couple of days in the sun will take care of the mildew stains. The UV will bleach 'em right out!
haven't tried the magic eraser yet...remember dont rub so hard with something like that to the point you scuff the "gloss" off the vinyl, or flatten the grain.
I'd give it a mellow once over and then out in the sun for a couple days...then you'll know what's left to be dealt with. You'll be suprised what the UV will do. Though this early depending on where you are the UV isn't that intense yet.