How to clean a light tan vinyl fence?
Assuming this actually is dirt and not bleaching by the sun and some confusion on which boards faced which way...

Go to Home Depot and rent a pressure washer.

Test the pressure washer on a scrap piece of fencing before you use it on the real deal - if you get too close you could blast a hole in the vinyl - but it will remove the dirt.  Quite amazingly so.  I've seen dirty old grey decking look like brand spanking new after a good pressure wash.