We have used hydrostatic pressure as a means for studying a resonant Raman mode observed at 47 cm(-1) in highly disordered, ion implanted, unannealed GaAs. The mode shifts weakly (-0.07 +/- 0.15 cm(-1)/GPa), supporting an identification of this band-resonant vibration as stemming from the breathing mode of the gallium vacancies, which are expected to be in high concentration. We measure a pressure coefficient of the longitudinal-optic phonon in these (5.5 nm) nanocrystals of GaAs to be 3.6 +/- 0.1 cm(-1)/GPa. The good agreement between our value and the pressure shift of this phonon in bulk GaAs implies that the bulk modulus is independent of size at least down to this size crystallite.