Its actually quite interesting when you vacuum the system since whatever moisture that is in the system actually starts to boil (crackle/snap) as you pull more and more vacuum. My rule of thumb when evacuating an AC system is that when you get to the snap-crackle-pop stage during the evacuation process (moisture returning to liquid from vapor under vacuum), I continue to pull a vacuum for about 10-15 minutes more and then call it good. You cannot achieve 100% vacuum (scientifically impossible), but you can achieve a vacuum state well below that needed to liquefy H2O at normal atmospheric pressures.