Vacuum-assisted gas atomization of liquid metal is explored. The investigation is motivated by observations of liquid metal atomization that indicate that secondary atomization is sustained over an extended distance from the nozzle tip. Increasing the velocity of the gas flow downstream of the nozzle exit by lowering the nozzle back pressure below ambient may therefore improve atomization efficiency. Supersonic jets grow in length when the nozzle back pressure is lowered due to an increase in the nozzle pressure ratio. However, since the nozzle mass flux remains fixed, any improvements in vacuum-assisted atomization efficiency will be realized without any increase to the gas-to-metal mass flow ratio, which is of interest both academically and practically as gas consumption can be costly. Small (25-kg batch) atomization runs were performed using an Al-Cu-Ni glass-forming alloy in which a high-mass-flow vacuum system was employed to maintain a sub-ambient chamber pressure over the course of an entire run. The powder produced in this manner was then compared to the conventional method without the vacuum system operating. Results demonstrate that atomizing into a partial vacuum decreases the frequency of the coarsest particles in the powder size distributions, leading to a narrower particle size distribution. Further, they underscore the importance of the axial length scale affecting secondary atomization that is related to, but not fully described by, the gas-to-liquid mass flux ratio. The present experiments point out a significant and unexplored parameter space that may be exploited to increase control over particle size distributions.