The intrinsic influence of gas sensing nature of S vacancy (V-S) and Mo vacancy (V-Mo) MoS2 monolayer plays an important vital role for sensing highly toxic gases. A systematic study was carried out to act its ability to be a gas sensor, which can be benefited from its high surface to volume ratio similar to graphene. The highly toxic gas molecules such as Cl-2, PH3, AsH3, BBr3 and SF4 was selected to examine the affinity with V-S and V-Mo MoS2 monolayer based on first principle calculations. To explore the sensing capacity and electronic property of V-S and V-Mo MoS2 monolayer the geometrical structures, adsorption energy, adsorption distance, charge transfer and density of states were analyzed and discussed. The results indicate that all the gas molecules are allowed to adsorb on V-S and V-Mo MoS2 monolayer through Van Der Waals interaction. Among all these gases, PH3 adsorption on V-Mo MoS2 monolayer exhibits relatively high adsorption energy of -1.8082 eV and charge transfer of -0.56 e, followed by the SF4 on V-S and V-Mo MoS2 monolayer showing enhanced adsorption energy of 0.9366 eV and -0.8052 eV and charge transfer of 0.029 e and 0.064 e respectively. PH3 and BBr3 adsorption on V-Mo MoS2 monolayer shows more change in the conductivity when compared to other gas molecules. Thus, V-S and V-MO MoS2 monolayer are producing beneficial results for sensing PH3, BBr3 and SF4 gas molecules than Cl-2 and AsH3.