Re: floating a closet floor
I would recommend the vapor barrier. As for fastening and gluing, you can probably get away with one or the other, you won't need both. Because it is a closet, who cares if it moves around a bit, you just don't want the joints separating (aesthetics only ). I would either glue the joints or throw a tack in them.
If you start in the back of the closet, face the tongue towards the door and drive your tacks at an angle through the tongue. Tap your next board in place and repeat. When you get to the door, custom cut your final piece and surface tack it along the leading edge, making sure that the nails will be hidden by the transition strip you'll be installing.
I suffer from CDO ... Its like OCD, but in alphabetical order, LIKE IT SHOULD BE!!!