Duration paint itself can be used on vinyl window shutters. I've used this product many times for that purpose. The store employee was correct though about possible warping if you're using black paint over a much lighter color. Using paint darker than the existing manufacturer finish can cause the vinyl to warp, especially if they're exposed to direct sunlight all day. The same thing can happen with vinyl siding painted a dark color. Black paint absorbs a lot of heat in the summer and gets very hot. This can cause problems with your plastic shutters. It doesn't mean that your shutters will definitely warp if you paint them black, but there's a potential risk, and the store might not back the paint warranty if it happens. The store should be able to help you with paint colors that are safe for use on vinyl. I don't recommend painting your shutters black, mostly for durability concerns. While Duration is a very durable product, black paint's going to fade a lot faster under the sun than other colors.