1. A system for determining a number of cylinders to operate in an internal combustion variable displacement engine, the system comprising: a processor, coupled to said engine, for determining whether the variable displacement engine should be operated on a fractional number of cylinders, with said processor inferring a desired fractional manifold vacuum for a half displacement mode when said engine is in any displacement mode; generating a vacuum recommendation signal representative of the variable displacement engine operating on the fractional number of cylinders, with the desired fractional manifold vacuum representing a vacuum that would produce a desired torque and desired emissions as if the variable displacement engine were operating on the fractional number of cylinders; inferring a desired mass air flow and a desired exhaust gas recirculation flow; and, generating a flow recommendation signal representative of the variable displacement engine operating on the fractional number of cylinders, with the desired mass air flow representing a mass air flow that would produce the desired torque and the desired emissions as if the variable displacement engine were operating on the fractional number of cylinders, and with the desired exhaust gas recirculation flow representing an exhaust gas recirculation flow that would produce the desired torque and the desired emissions as if the variable displacement engine were operating on the fractional number of cylinders; wherein said processor controls the operation of the variable displacement engine responsive to said vacuum recommendation signal and said flow recommendation signal both being within predetermined ranges for enabling the operation on the fractional number of cylinders.