Vapor sensor is an important application for conductive polymer composites (CPCs) chemiresistors which is based on the typical percolation behaviors of CPCs. When towards organic vapor, the conductive networks in CPCs would be changed, which results in the electrical resistance changes. Based on this novel property, they can be used as e-nose, human health monitor, environment detector etc. In this study, we explore the microstructure of polycarbonate/polystyrene/multi-walled carbon nanotube (PC/PS/MWCNT) blend composites with different composition prepared by melt mixing. SEM exhibits a sea-island morphology for blend composites containing 0.75 wt.% MWCNT with a PC/PS weight composition of 70/30 (C70S30M0.75), while the weight composition of 50/50 at same MWCNT content (C50S50M0.75) forms a co-continuous structure. Based on surface tension parameters of components, it is assumed that MWCNT preferentially locate within the PC phase. This can also be proven by selective extraction experiment: a clear solution can be obtained using cyclohexane, which is used for PS extraction. Vapor sensing performance is related with the swellability of the PC phase with MWCNT network. Vapor of good solvents for PC (dichloromethane and acetone) show a poor reversibility for all CPCs due to the strong interaction between vapor and PC, but C70S30M0.75 exhibits the highest sensitivity among the three CPCs. Vapors with moderate PC solubility (ethyl acetate and toluene) display similar sensing response as good solvent vapors, however, C50S50M0.75 shows an excellent reversibility due to large interface which facilitates the evaporation of vapors. Finally, the PC/PS blend CPCs are also suitable for sensing poor solvent vapors for PC (e.g. cyclohexane) due to the forced MWCNT network changes by the swelling of PS phase, even if the MWCNT are located within the PC phase. Such novel vapor sensor provides a new possibility for sensing vapors with wide ranges of solubility parameters.