Keeping your vinyl siding clean is the best way to preserve your investment. This is something that you can do easily yourself. In very few cases, to maintain the warrantee, you would have to hire someone to come out periodically to clean your vinyl siding.
However, if this is not the case, and your home is mostly or completely covered in vinyl siding, it would be a good idea to get a pressure washer. These are quite easy to use and will make the job much easier. You can purchase a reasonably priced pressure washer at your local hardware store and it can be used for many things other than washing your house. Most of these pressure washers use a mixture of gas and oil to make it run. Make sure you consult the manual for the correct mixture of oil to gas for your machine. You can find the oil right there in the lawn and garden section of the store, and sometimes they will actually have it right beside the washer you are buying. You don't want to spray the water too forcefully on the vinyl siding because you want to avoid getting water behind it.
Getting water behind the siding could result in mildew formation and severely shorten the life of your purchase. There are specific cleaners that can be purchased to clean, but that is not necessary, regular dish soap will do just fine. (Unless your warrantee depends on using a only a certain type of cleaner) You will want to make sure that the soap is completely rinsed off though, or else you will be left with a white and slightly sticky residue.