Summarize:

If you've cleaned your fence with vinegar or detergent and it still appears dirty, you may need to try a specialized cleaning solvent. Some solvents are formulated to help remove dirt and grime from vinyl fences. Visit your local home improvement store and look for products specially formulated to clean vinyl siding and vinyl fences. Specialized solvents may come in their own spray bottles or you may need to pour them into a garden sprayer or similar device. If you've used another method and still have trouble cleaning your vinyl fence, you may want to consider a bleach solution. Mix 1/3 cup (80 ml) of laundry detergent, 1 quart (.94 l) of bleach, and 1 gallon (3.78 l) of water. Pour the solution into a garden sprayer and spray down your fence. Avoid using this mixture on non-white vinyl. If other methods have failed, you may want to consider using a pressure washer to clean your vinyl fence. High pressure water will help dislodge gunk, debris, or stains. Rent or purchase a pressure cleaner at a local home improvement box store or hardware store. Try using high pressure water without chemicals first. If you need to use chemicals, the pressure cleaner may have a small tank where you can pour a chemical solution. The pressure cleaner will automatically mix the chemical with the water as you spray it. If you've tried one method to clean your vinyl and it still appears dirty, you may need to repeat or try another method of cleaning. Ultimately, vinyl fences exposed to the elements will collect a lot of dirt and may require substantial time to clean. Don't be afraid to try several methods. You may need to use one method a couple of times to get the desired effect.
Use a specialized solvent. Spray a bleach solution on your fence. Try high pressure water. Use different methods and repeat as necessary.