A composite polymer electrolyte film was prepared by dissolving polyethylene glycol (PEG) with different molecular weight in acetonitrile, and vapor-induced response behavior was investigated upon exposure to various chemical environments. The effect of lithium concentrations on ionic conductivity and response was discussed. The surface microporous structures and vapor sensitive conductivity of the films in the case of poly(vinylidene fluoride) (PVDF) were examined with the PVDF content changed. The crystalline and micro-phase isolation behavior were characterized by a differential scanning calorimeter, an environmental scanning electron microscope, a polarization microscope and a wide-angle X-ray diffraction. The experimental results indicated that PEG/Li+ salt composite films exhibited preferential responsive characteristics. The responsivities to ethanoic acid, chloroform, and acetone vapors were enhanced with molecular weight of PEG increased. The conductivity was increased at a higher lithium salt concentration, and also enhanced with PEG content increased, while the responsivities decreased. The formation of microporous structures on the surface of the mixed PEG/PVDF composite films enlarged their specific area and strikingly improved the responsive performances. The changes in conduction behavior were explained from the viewpoint of the swelling and free volume theories as well as a hydrogen bond interaction, combined with the structural and morphological analyses. The introduction of an ionogenic matter also has an important effect on ionic conductivity and responsiveness.