Pressure testing with a nitrogen/refrigerant mix can at times find leaks that a vacuum test can't. Also, with the vacuum test, the tech won't know WHERE the leak is, only that he's not holding a vacuum. So if he detects a leak with a vacuum, what does he then have to do? Gas up the system and go hunting. Might as well do that first, find all leaks possible, and THEN pull a vacuum. Always better to do things in the right order vs. doubling back and eating up time and money.
If a system is tight, a tech looks for the following reality when pulling a vacuum:
System should hold at 500 microns or lower. If microns rise, level off, and hold, system has moisture in it and requires more evacuation. If system micron level rises to atmospheric, there's a leak.
Freeze stat is not the answer. Allowing a system to run low on refrigerant, even if it still appears to cool the house, puts the compressor under stress. If the compressor can't be cooled sufficiently by suction gas, and oil return to the compressor becomes spotty, compressor life will be short and it will die an ugly death.