Swanstone does not require a vapor barrier but it is so cheap do do just go ahead and lay down 15lbs felt or visqueen. However, I never have and have had no issues. he biggest failure installing a Swanstone is forgetting to grout the base in with Structolite or dry mixed drywall compound or even flooring thinset. A slight flex can happen to the floor of the base and that will result is noise and eventual leaks at the drain. Didn't you get a booklet like this one? http://www.swanstone.com/images/link.../BK-326072.pdf