If you go with the vinyl trim look for the type that has the finished edge on all sides. Some have open foam core showing and it tends to mold up.
I'd give it a light sanding before painting to give it some tooth for the paint to hang onto. Also only use 100% acrilic latex paint or it will peel.
Use stainless steel trim head or ceramic trim head screws to attach it. Finish nails tend to pull through as the vinyl expands and contracts. Just fill the heads with 50 year latex caulking and wipe the face of the material with a damp sponge making sure there's 0 caulking left on the face of the materail or it will show when painted.