The vibrational sum frequency generation spectroscopy (VSFG), one of the second order nonlinear optical tools, has recently become a versatile characterization probe for the molecular interfaces owing to its unique surface selectivity and submonolayer sensitivity. In the past years, the applications of the VSFG technique have expanded into various research fields including biological sciences, environmental chemistry, heterogeneous catalysis, electrochemistry, and etc. However, the proper interpretation of the VSFG data is not trivia. This is partly due to the complexity of the VSFG selection rules. In addition, the complicated chemical and physical nature of the interfaces also made the straightforward explanation of VSFG data difficult. For example, the air/liquid interface of the water-methanol binary interface is among the first liquid interfaces that have been studied by the VSFG. It has been observed that the VSFG intensities from the methanol molecules first increase then decrease along with the increase of the methanol concentrations, the reason of which has been debated for more than 20 years. In this presentation, we will analyze all of the possible factors and provide the correct explanation for this nomonotonic trend of the VSFG intensities as the function of methanol concentrations. We will also show in this presentation our recent results on the polarization selection rules of VSFG that can be used to interpret the newly observed spectral splitting for the model systems of the biomembrane systems.