Can you stain a vinyl deck?

Yes, you could stain vinyl, but I wouldn't. Actually, you could stain nearly anything it just wouldn't always come out the way you hoped. Vinyl would not take a semi-transparent stain the same way wood does. There are no pores or grain patterns to absorb the stain. The results would be a little odd looking. Every nick, scratch, blemish or other imperfection would stand out 100 fold. Your best bet, if you just want to change the color of your deck, would be to apply a solid, non transparent exterior stain or a quality exterior paint. The trick in doing either of those is in the prep. Because the vinyl will oxidize over time it must be removed or sealed out completely before applying a finish. If you take your finger and rub it on an area that gets a lot of sun, you will see the oxidization on your finger. First thing to do is power wash the deck. After it dries you apply a coat of exterior primer/sealer and let that dry. Then you can apply the color of paint you desire and you shouldn't have any problems. Remember, if you want your new deck color to last don't take short cuts in the preparation. Good luck!