You use a vacuum pump to boil off moisture in the A/C system not just to vacuum air out the system.
So lets pick on Harbor Freight. They sell a air operated vacuum pump for less than $20. Item #96677 Per the web site it has a vacuum level of 28.3" of mercury at sea level.
Now lets look at when water will boil.
Water boils at 212 deg at sea level.
If we increase the pressure on the water we can raise that number such as in a car radiator system with a pressure cap.
If we lower the pressure to a vacuum we can lower that boiling point pressure.
At a vacuum level of 28.3" of mercury the air temperature would have to be about 95 deg for water to boil.
So even if you have a big compressor that will put out enough air; and even if the vacuum pump works as good as they say in the owners manual; the outside air temperature would have to be 95 deg or more for it to pull a big enough vacuum to make the moisture boil. If the water does not boil you are wasting your time vacuuming out the system.