A vac check is OK to test for gross leaks but 2 things will occur when under vac:

Blind bolt holes / trapped voids / oil will continue to gas out making the vac level change for the worse.

Compressor seals really are not rated for vacuum.

The best way to check for system integrity is to fill with a dry gas. Usually Nitrogen is used because it is convenient in an industrial setting however it is a large molecule and the rate of leak will be reduced.

A way accessible to anyone recharging their system is to: ( this works better with R 132 due to the cost of R12 ) Leave the AC and engine off. With the freon can in an upright position to draw gas, open the valve and let system fill. The pressure will equalize at some point. Shut off the can and note the pressure. Disconnect the gauges and let is sit for a few hours / days then check pressure again. Be aware that freon pressure is low at cold temps and rises at high temps so take that into account.

You won't be using much freon doing this as you are only adding gas, not liquid and the AC system isn't turning your gas into liquid. Next time I charge a system I'll weigh the can and see what it takes to gas a system.

If you charge a system with a running compressor and get liquid in the system, your engine off pressure will never drop until all the liquid has boiled off.

Freon ( and any gas that has been turned into a liquid like propane , CO2 , nitrogen , oxygen. ) will have a vapor pressure related to temperature. Take some liquid freon, dump it into a bottle and put the cap on. The pressure will rise then stop. At this point forces that want the freon to boil off into a gas are countered by the gas pressure in the vessel. This is an equilibrium state.

If you add or draw liquid off, the pressure will be the same once things stabilize. So , this is why using freon gas only is a good way to test for a leak. Any leak in the system will result in an immediate loss of pressure.