AlSi9Cu3(Fe) aluminum alloy specimens were produced by conventional high-pressure die casting (HPDC) and vacuum-assisted high-pressure die casting (VPDC) processes under atmospheric and 3 different absolute pressures of 170 mbar, 90 mbar, and 70 mbar. The influence of absolute pressure in the die cavity on the porosity and mechanical properties of the die castings were investigated and compared with the traditional casting method. The life data analysis was applied to deal with the variation in mechanical properties of the die cast flat tensile specimens. The porosity of castings was assessed on the basis of X-ray observation and density measurements performed by Archimedes method. The fracture surfaces were examined by scanning electron microscopy (SEM), and the chemical composition was measured by energy dispersive X-ray analysis (EDX). The investigations proved that the volume of gas porosity and the pore sizes in the castings can be significantly reduced by using vacuum assistance during the die casting process. Based on the data presented in the study, the porosity reduced from 1.10% at an atmospheric level to 0.47% at 70 mbar, which corresponds to 57% reduction. The boundary condition of the present investigation is the porosity content higher than 0.40% and lower than 1.10%. As a result, the density and the mechanical properties, particularly the tensile strength (10%) and elongation (50%) were significantly improved. The specimens contained smaller pores under lower pressure. Meanwhile, the shape of pores is found to be also an important factor, affecting the mechanical properties. In general, higher vacuum degree contributes to the reduction the porosity, which would be the basis to improve the mechanical properties of die cast parts.