Many vinyl side manufactures discourage power washers. It pushes too much water under the joins and causes other moisture problems. I have been using a pump sprayer with oxygenated bleach (sodium perborate). 1 small scoop and a gallon of warm water sprayed on the surface, then take a rag and wipe it off. Hose off with clear water...goes really fast and not much scrubbing.
Oxygen bleach is a great cleaner and cheap---bought a 16 oz container at Dollar Tree for yes a $1. I also used it on the old doors that have been stored in our shed for the past 10 years. I'm planning a couple of used door projects with them.