All I ever use on my vinyl siding is a 50/50 mix of bleach and water. With light mildew or algae just let it sit a couple of minutes then hose it down. With heavy stubborn mildew or algae spray it, let it sit a few minutes, spray it again and then sweep it down with a wide push broom, then hose it off.
Mine always looks like new after.
PS. Vinyl siding instructions say to never use a pressure washer on it because water gets behind the siding causing more rot and mildew. Most people never read those instructions.