Vinyl siding is tough, but not indestructible. If a falling branch or a well-hit baseball has cracked a piece of your siding, you can make it as good as new in about 15 minutes with a $5 zip tool (available at any home center) and a replacement piece. It's as simple as unzipping the damaged piece and snapping in a new one.
Starting at one end of the damaged piece, push the end of the zip tool up under the siding until you feel it hook the bottom lip (Photo 1). Pull the zip tool downward and out to unhook the bottom lip, then slide it along the edge, pulling the siding out as you go. Then unzip any pieces above the damaged piece. Hold them out of the way with your elbow while you pry out the nails that hold the damaged piece in place (Photo 2).
Slide the replacement piece up into place, pushing up until the lower lip locks into the piece below it. Drive 1-1/4-in. roofing nails through the nailing flange. Space them about every 16 in. (near the old nail holes). Nail in the center of the nailing slot and leave about 1/32 in. of space between the nail head and the siding so the vinyl can move freely. Don't nail the heads tightly or the siding will buckle when it warms up.
With the new piece nailed, use the zip tool to lock the upper piece down over it. Start at one end and pull the lip down, twisting the tool slightly to force the leading edge down (Photo 3). Slide the zip tool along, pushing in on the vinyl just behind the tool with your other hand so it snaps into place.
It's best to repair vinyl in warm weather. In temperatures below freezing it becomes less flexible and may crack.
The downside of replacing older vinyl siding is that it can be hard to match the style and color, and siding rarely has any identifying marks. The best way to get a replacement piece is to take the broken piece to vinyl siding distributors in your area and find the closest match. If the old vinyl has faded or you can't find the right color, take the broken piece to a paint store and have the color matched. Paint the replacement piece with one coat of top-quality acrylic primer followed by acrylic house paint—acrylic paint will flex with the movement of the vinyl.