IMHO a vac test is a total waste of time and money.
The main component in the system is the COMPRESSOR which causes an INCREASE in pressure within the system, this stresses seals and fixtures from the inside to out, and any such stresses leads to leaks, sometimes quite small, but the total content of gas gradually decreases.
A vac test causes a decrease in pressure, and the atmosphere will try and enter the system .. in fact it sucks down on all the seals and fixtures and unless the hole is the size of the planet (in which case all the original gas has gone) it has a tendency to reseal any stresses as it is working in totally the wrong direction. Of course, when the system passes they can regas it and take your money .. when you actually have a leak somewhere that you need fixing.
A nitrogen pressure test is far more accurate but is also more expensive ..
