In this study, the high-pressure behavior of monoclinic vanadium dioxide (VO2 (M-1)) was revisited using infrared reflectivity (IR) spectroscopy, Raman spectroscopy and in situ synchrotron X-ray diffraction (XRD) up to 64.7 GPa. Upon compression, VO2 (M-1) follows the expected the structure transition sequence, M-1 -> M'(1) -> X, and we found that the structural transition from M'(1) to X phase is completed at about 59 GPa. Moreover, our IR data demonstrated that the M'(1) phase is a semiconductor within the pressure region of 11.4-43.2 GPa and became metallic with further compression, and that the X phase is metallic. Further analysis suggests that the pressure-induced metallization (PIM) of the M'(1) phase is associated with electron-electron correlations, while the PIM from the M'(1) to the X phases is relate to structural phase transitions. These results provide further insight into the PIM of VO2(M-1).