Sometimes your vinyl siding getting vandalized, sometimes you accidentally splashed a little paint on it while painting something else. Here are some tips on trying to remove the unwanted pain.
First try common household laundry detergent. Mix about a cup of it with water and scrub with a brush.
Goof off, or goo gone are excellent products for removing almost anything from anything. They come in both pourable and aerosol containers. I would suggest spot testing these products in an inconspicuous place and leaving them for a little while to make sure they are not going to damage the surface. If it does not damage the spot tested apply it directly to the paint and try to scrub off, rinsing the entire area afterward with plenty of water.
Try dish soap and baking soda. Mix them to form a paste. Apply the paste to the paint and gently scrub them with a soft rag.
Paint thinner or acetone is said to remove paint stains but may also dull dull the finish on your siding. I suggest thinning it with 1 part thinner to 10 parts water and scrubbing on the stain only after testing a small inconspicuous area. Paint thinner can be hazardous to other surfaces so be careful not to spill it on other surfaces and wear gloves to keep it off your skin.
If none of these work try renting a pressure washer but be aware that it may make the rest of your siding look dirty as it accumulates dirt and grime over time.