Actually, they make a vinyl siding cleaner you can add some bleach to and it works great.  

The eve's and gables on my house has siding on them, got tired of keeping them painted, and I use that cleaner.  I got it at HD or Lowes.  I spray it on with a 3 gallon, pump up sprayer.  My pressure washer has a chemical siphon port but I don't use it.  Don't won't to take the chance of chemicals getting the pump.  Chemicals and pumps don't do well together.

8 hours ago, Jig Man said:

If you have vinyl siding you can clean it without a power washer in a fraction of the time with a 50/50 solution of bleach and water.  Just spray it on, give it a few minutes to work, then rinse it off, done.  I can do in 30 minutes what used to take half of a day.

My house is so small I can reach all my siding with a rag and a step ladder haha