TOT,
I beg to differ regarding your assertion that there is a vacuum in the inlet manifold.
There ispressure in the manifold. It may be less than atmospheric, but it is a pressure none the less. In a normally aspirated engine this somewhere in the range of approx. 10"Hg to 27"Hg, depending on power setting.
The only time it would be a vacuum would be if all gas was evacuated from the chamber in which case there would be no mixture to enter the cylinders.