Ther should NOT be a vapor barrie on the exterior of the house. As Jim ATS stated, Tyvek or comparable products help to reduce air infiltration w/o being a vapor barrier. Jim is also correct on trying to stop the leakage at the source. Foaming around the windows will help, but use a minimally expanding foam, not the expanding foam, in such areas to reduce the chance on warping the jambs. Another thing is by stopping it where you notice it, perhaps a thin bead of a clear caulk aroung the moulding on the inside, or gaskets in your outlet & switch plates on exterior walls.