Personally, I don't build or remodel homes for a living but I've never seen a vapor barrier in the 'attic'. I'd suspect the reason is is because you want moisture to get out of the house if it gets in. Should the inflow be greater than the outflow, you could conceivably get condensation and such. I know a tight house is OK as long as no one is asmatic or has respiratory problems or the like. But, moisture needs to get out. Not every home has a dehumidifier.
This is a question beyond me but I thought I'd share my 'thoughts'.