Like anything exposed to the elements, vinyl siding will get dirty. However, in most cases, a good rainfall or an ordinary garden hose will do wonders on the normal everyday dirt.
If the rain and a hose are not successful, use a soft bristled brush, such as a car washing brush, with a solution of water and vinegar, or any of the specified ‘siding cleaners’ available at your local hardware store. Laundry bleach can be added to this solution to address mildew.
Scrubbing from the top down will help to reduce streaking.