Try using Meguiar's Heavy Duty Vinyl Cleaner. I bought some after getting caught in a rain/windstorm at a gas station one year at Carlisle. The wind/rain blew dirt & grease from the tops of the gas pumps all over my white vinyl top. It was literally disgusting. I went back to the fairgrounds & bought some along with a bristle brush. It worked awesome on white!

You could also try Spray Nine. That also works great!