Before a vacuum is pulled, the leak test should have already been done. When the lines are brazed, there will be contaminants to form and must be removed. After this is done, the nitrogen is then used to leak test the system. I have never used a vacuum pump to check for a leak., by the time it is on a vacuum, you are clearing your area and preparing to charge the unit. I guess it is all in training, but checking for a leak when vacuuming is a poor way of doing an install. It is a given removing impurities that will extend the life of the compressor; this is the reason you should flush the line set.
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