Vapor barrier comes in various forms and is highly recommended for all steam shower applications.
Without a doubt, vapor barrier should always be used in steam shower applications. This provides additional protection for your walls in case there is any absorption of steam through your wall materials, or in your case, tile and grout. Check out the link below to learn more about wall construction for steam shower environments.http://www.steamsaunabath.com//catalog/all_about_steam_rooms_walls.php#walls
Here is a new product which does a great job of vapor proofing