Like anything else a tester is no better than the knowledge being used to test. Vacuum testing usually involves either determining leaks in servo controlled systems or evaluating levels/conditions of vacuum for engine diagnosis or vacuum system performance.
The type of tool you seem to be looking at is used to create VACUUM not pressure; actually the lack of pressure. A good tool should get you close to perfect vacuum 29.xx inches of mercury (around thirty feet of water). Any system running on vacuum can be pumped as low as the pump can take it. It usually is not a question of how low but does it stay. In other words if you are checking the fuel shut-off servo on the back of the injection pump one would pull 15-20 inches of vac and let it hold. It should be stable and stay at the vacuum pumped indefinitely.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician