Move the vinyl seat coverings to a sunny location. Allow the seat coverings to sit in the direct sunlight for a couple hours or until dry, if wet. Sunlight is a natural mold and mildew killer.
Brush visible mildew spores off your vinyl seat coverings using a stiff-bristled brush. Brush away from you so mildew spores don't land on your clothing.
Mix 1/8 cup each dishwashing liquid and bleach in a large bucket with 2 gallons of warm water. Put rubber gloves on your hands to protect them from the bleach solution and mildew, then stir the solution with your hands to mix.
Wet a cloth in the bleach mixture. Wipe down your vinyl seat coverings with the wet bleach cloth, paying special attention to the mildew-stained areas of your seat coverings. Rinse your clean cloth with plain water as you pick up the mildew stains, then apply more bleach solution and continue to wash your seat coverings until the mildew stains are removed.
Rinse your seat coverings on both sides and along the edges using your garden hose. Rinse thoroughly to remove all the bleach solution so it cannot weaken the seat-covering material.
Dry your vinyl seat coverings in the sunlight to ensure all moisture from the cleaning is removed.