Can I purchase just one piece of vinyl siding?

There is a massive range of vinyl siding colors available so repairing and/or replacing vinyl sliding should not be a problem. Most do-it-yourself stores will have a leaflet with the vinyl siding colors which you can use at home to color match your sliding. Alternatively the websites will have vinyl siding colors as well, just be aware that your they might not be exactly true to life. The best match for vinyl siding colors would be to take a piece with you to the store.