what works well is krud kutter house wash.you don't need high pressure to wash vinyl siding.but using a pressure washer is ok to rinse off the higher up areas that a garden hose alone won't reach.
follow the directions.apply the cleaner let it set for the specified time then rinse it off.
don't work in the direct sunlight
and(trust me on this one)start from the bottom and work your way up.
i like the krud kutter because it doesn't harm the grass or plants but it does say to wet down the grass and foilage before you start.
they got it at home depot.