That's too much of a drop. A tight system will show almost no movement overnight - perhaps a needle's width on the low side guage.

However. Has the system been run since the condensor/compressor/dryer were replaced? The 'used' oil can hold a surprising amount of refrigerant, even while open to the atmosphere. Your drop in vacuum may be due to refrigerant migrating out of the oil under vacuum.

Basically, we can't tell if your system is tight or not. Vacuum it for several hours - perhaps 4 - then let it sit overnight. If you get a lot less vacuum loss, that's a good sign.

- JimY