  \textit{In situ} high-pressure Brillouin light scattering experiments along
loading-unloading paths are used to investigate the compressibility of vitreous
silica. An accurate equation of state is obtained below \SI{9}{GPa} using sound
velocities corrected for dispersion. Conversely, huge inelastic effects are
observed in the range \SIrange{10}{60}{GPa}, unveiling the reversible
transformation from the fourfold-coordinated structure to the sixfold one. We
find that the associated density changes fully correlate with the average Si
coordination number. Decompression curves from above \SI{20}{GPa} reveal abrupt
backward coordination changes around \SIrange{10}{15}{GPa} and significant
hysteresis. Further, contrary to common wisdom, the residual densification of
recovered silica samples can be figured out from the pressure cycles.
