Vacancy sensors in place of occupancy sensors

Can vacancy sensors be used in place of occupancy sensors in appropriate spaces? Vacancy sensors, like occupancy sensors, will turn the lights off when the room is deemed vacant. Unlike occupancy sensors, they require the user to turn the light on when entering the room (occupancy = automatic on/automatic off; vacancy = manual on/automatic off). This could result in potentially more savings as compared to occupancy sensors because they require the user to turn the lights on, which the user may not choose to do in some situations (say, walking into his/her office briefly to grab something).