Using N+2 as the molecular probe, we investigate the ionization behavior of the nitrogen molecule buffered in different rare gases （helium and neon） by optical heterodyne velocity modulation spectroscopy. The spectral intensity of an individual rovibronic line of N+2 is proportional to the product of the velocity modulation index and the concentration of N+2. Therefore, the relative ionization depth can be deduced via measuring the spectral intensity in different buffer gases.