The vibrational sum frequency generation (SFG) spectra of gas - liquid interfaces of NaCl and NaI aqueous solutions are computed and analyzed by molecular dynamics (MD) simulations using a flexible and polarizable molecular model. The MD calculations have reproduced the experimental features of SFG spectra, including observed perturbation on the Nal spectra in contrast to little perturbation on NaCl. Analysis of the nonlinear susceptibility revealed that the intermolecular correlation has a significant contribution to the vibrationally resonant amplitude, which largely distorts the generally accepted relationship between the SFG intensity and orientation of individual molecules. In Nal solutions, modest enhancement of ssp intensity in the 3400 cm -1 region is thereby elucidated by this mechanism. Regarding the sps spectra, three spectral components are assigned and elucidated. Calculated remarkable enhancement in the 3400-3800 cm-1 region for NaI solutions is found to be sensitive to the electric double layer structure. It is also revealed that the sps intensity is augmented by the intermolecular water - water correlation effect.