To improve fuel economy, some types of vehicles such as hybrid electric vehicles (HEVs) may provide regenerative (regen) braking, in which kinetic energy is converted by an electric machine into storable energy during braking and then made available for propulsion. In regenerative braking systems, a booster may use vacuum pressure which is generated by a vacuum pump to increase mechanical regenerative braking power. The vacuum pump used in a regenerative braking system, however, may consume unacceptable quantities of energy. Therefore, a vacuum accumulator system and method is needed in which the run time of a vacuum pump, and consequently, energy consumption of the pump, is reduced by the storage and use of excess engine vacuum pressure to augment the vacuum pressure which is distributed to the booster in a regenerative braking system.