Derek's approach would probably be much better than what I did.

I tried to pretend that the 6 mil vapor barrier was a temporary roof. Not so! The nail holes in the vap barrier allowed moisture in, which was then trapped between the t&g and the vapor barrier and never dried out - warping some of the t&g boards. Also, after about 8 weeks, the vapor barrier disintegrated in the sun light and had to be reapplied in some areas. I should warn that the worst instance of warping (the one that required ripping and trashing 200 square feet of beautiful prefinished 7/8" oak t&g) occurred where I used 90lb role roofing as a temp roof, and the roofing nails did not self-seal where I thought they would. Like one of the slate books says, "a matchstick sized hole in the wrong place (e.g. a valley) can leak gallons of water in a single rain storm." It probably only takes a few spoonfuls of trapped water to warp t&g and/or ruin the finish.

tar/felt paper would probably have worked better - I think it would have let some of the moisture out if it had leaked. But then you are increasing the combustibility of your ceiling, and the tar paper genuinely stinks when it gets hot. What to do?