If you only have dirt on the walls (or dust and such), then just water is needed.
If you have mold/mildew, you can buy Vinyl Siding Cleaner for about $5 a gallon. Read the directions for the amount of area it will cover if your washer has a siphon on it for this purpose. (As an aside, 1 square of vinyl = 10 x 10 = 100 sq. ft. You'll need to measure up your siding to see how many squares, or sq. ft., you have ...)
If you're simply wetting and washing the walls by hand (assuming you're cleaning off mold/mildew as well as dirt), the container should say how to do it. Depending on the amount of M/M you have, you may have to go stronger. On my particular siding, I have white too, and the BACK of the house (the north side) had the worst M/M. I had to scrub it by hand because neither plain, old water (under pressure) nor a 'mixture' (under pressure) would work. I had to mix a solution in a bucket and scrub it by hand. I used one of those bristled bathroom brushes. Be SURE to wear rubber gloves because whatever you use will eat at your hands!!!
You can try a homemade solution of bleach and water but do realize that this mixture can kill plants and such. However, in the Fall, you can get away w/it a bit more liberally since 'stuff' is dying anyways (unless you live in a 'nice' climate year-around.) Start w/1 cup bleach per gallon of water. There are other products that are biodegradable such as NewAgain or OxygenBleach
My best to ya and hope this helps.
Jay J -Moderator
PS: Clean from the BOTTOM, up; NOT from the top, down. If you do the latter, you'll get streaks. Rinse all the way down as you work your way up. This way, you don't leave any cleaner on the siding and you don't get streaking. And be sure to wet the wall FIRST w/plain water BEFORE you apply any cleaner or mixture ...