Vacuum pressure is just the amount of pressure negative or positive in the intake section of a car, with N/A cars you are always negative numbers meaning that you engine is acting as a vacuum and sucking the air that it needs in just like a household vacuum suckes up the dirt, but on boosted cars you have negative vacuum at idle but then your power adder kicks in it forces air into the intake/engine this is a positive vacuum.
so negative vacuum = n/a car and positive vacuum = boosted car
I believe thats where superchargers got the name blowers from is the fact that it is blowing air into the engine versus the engine sucking the air in..
