It's become very important to understand the difference between a vapor barrier and an air barrier.
If we completely seal a polyethylene sheet on the warm side of the insulation, we call it a combined air/vapor barrier.
But it's possible to put the vapor barrier on the warm side of the insulation, without sealing it air tight, as long as we've created an uninterrupted air barrier someplace else, in the wall or ceiling. House wraps are now often used on the outside of the house to stop air movement, in combination with a vapor barrier on the inside of the house to stop moisture diffusion into the wall.
Watch the video above that I made with the help of the Canadian National Research Council to see this more clearly.