Leak Detection While in a Vacuum
This is not a recommended leak test procedure.

If the system is in a vacuum, and there is a leak in the system, the vacuum gauge will start to rise. Pulling in the atmosphere you just tried to remove. And, it is very hard to find the leak.

Also, when a vacuum is used for leak checking, it is only proving that the system will not leak under a pressure difference of 14.696 psi. (Depending on elevation) If the entire atmosphere is removed from a system, there is only the atmosphere's pressure. When checking for a leak using a vacuum, the technician is using a negative pressure of only 14.696 psi. The systems operating pressures are much higher.

Using a vacuum for leak checking may also hide a leak.
Example: If a tiny hole at a solder connection has a flux build-up or dirt and oil build-up, the vacuum will tend to pull this into the tiny hole, and may even hide it to a point where a deep vacuum is achieved. When pressure is applied to the system, the leak will show up, maybe long after the technician has left the job.

The Best Leak Test is the Standing Pressure Test

To perform this test, put a small amount of refrigerant is the system up to about 10 psi, and then pressurize the system to 150 psi using nitrogen. The refrigerant can be detected with a leak detector. If you do not have a leak detector, skip the refrigerant and leak test with soap bubbles, a rich mixture of soap and water.

Atmospheric Pressure = 14.696 psi at sea level, at 70 degrees Fahrenheit.