Vinyl gutters require a more complex process of painting them because paint hardly adheres evenly on vinyl surfaces. But by using the right materials and following the correct steps, you can finish the paint job in a few hours.
It is easier to paint them if you take the gutters off.
You want to make the paint to adhere more evenly, but you do not want to rough up the surface of the gutters. So just gently use the sandpaper on the surface without over sanding it.
If you can’t remove the gutters from their place, you can always use a sprayer, or maybe you prefer a brush. Only use acrylic paint because light paint adheres better to vinyl. If the first coat of paint doesn’t seem enough, you can apply a second one when the first one has dried.