To remove dried concrete from vinyl siding, use a concrete dissolver like Speedy-Clean Concrete Dissolver. As of 2014, this dissolver is advertised as a 100 percent biodegradable green cleaner that uses acids to break down the concrete and stop it from sticking to the surface it is on. According to the manufacturer, this product is safe for use on siding.
It may be helpful to wet the concrete, which helps to soften and loosen it from the siding. If water alone doesn't work, try using a spray bottle full of water with a few drops of dish soap to wet the concrete. Then, scrape off the concrete gently with a putty knife. This method is recommended for use on windows, and it should be safe for vinyl siding as well, as long as one scrapes gently.
To prevent concrete from splattering onto other surfaces, cover windows and siding before starting on any project that involves concrete. If the concrete does splatter, try to remove or clean it as quickly as possible before it hardens.
To improve the efficiency of concrete dissolvers, consider adding water to area being addressed. If the concrete is wet throughout the removal process, it makes it easier to work with. However, do not do this if it contradicts the manufacturer's instructions.