We very carefully used a heat gun to soften the adhesive and then the vinyl peeled right off. Then used acetone (have window open, fan running) to remove the residue.
Used two part polyurethane marine deck paint to cover sufaces. A royal pain (paint is very thin, hard to keep from getting runs/sags - use several very light coats) but it has held up great and looks very nice.
Others have had great success using POR-15. I used it after I did the bathroom on the exterior a-frame and, in hindsight, would probably use it instead of the marine paint if I were to redo the shower. It is easier to apply and forms a rock hard surface that is extremely easy to keep clean.
POR-15 does have a very short shelf life, however, and while seemingly pricey it is not more expensive than marine poly and a little goes a long way.