Since we're talking northern climate (think Detroit / Buffalo / Toronto) and since there is already a vapor barrier on the back side of the interior drywall, I wouldn't think that its a good idea to have a second vapor barrier anywhere, especially closer to the exterior.
The exterior sheathing is some sort of drywall / gyprock or sheetrock, presumably exterior grade, and it's covered with a couple layers of some sort of thin black paper (paper has a waxy or oily feel to it, tears easily).
I'm thinking of taking the paper off, sealing the seams between the exterior drywall panels, putting up some 2" expanded polystyrene 4x8 panels, and then covering with 7/16 OSB. I figure the OSB will be able to sandwich the foam good and tight against the drywall, removing air gaps, and giving a good attachment surface for the siding. This is for the second story, the ground floor has brick exterior.