Vacuum pumps that installed in freeze drying equipment is not for “Suck” the vapors, its function is maintain the vacuum environment.
The driving force in freeze drying is the vapor pressure difference between the sample temperature and the condenser temperature. The water molecules will move to coldest area, that is the condenser.
A simple example: when you take something out from the freezer in your home, something like fog occurs, that is sublimation, but there is no vacuum pump.
The vacuum degree, is not the ultimate the better. This is because: under deep vacuum, the heat is hard to transfer to raw product which help sublimation. Thus, the ultimate pressure will slow your freeze drying speed. However, in secondary drying stage, bound water remove request a deeper vacuum level.