All concrete slabs below habitable areas/spaces should be provided with a vapor barrier. A concrete slab sitting on grade is nothing more than a very large tampon when it comes to wicking moisture out of the sub-soil. There is a jurisdiction in my area that has for years required a 2-ply asphaltic hot-mop VB below garage slabs due to the propensity of folks for eventually converting their garages to family rooms and/or sleeping rooms.