Without totally rereading the entire thread, I'm not sure why you are wanting to do a vacuum test. You would have to recover existing charge and then evacuate to do a vacuum test. A vacuum test, as you call it, will only find a large leak. We used to call that a gross leak test.

Since it has a partial charge, why not add UV dye and maybe an ounce of oil and charge the system. Then observe with a black light while it is cooling properly. That way you can get condensation water and shine the black light in that water. If there is dye in it then you know the evap is leaking. If it's leaking elsewhere you can then find that leak.

Once you spot the leak, THEN you can recover the R12, repair the leak, evacuate and charge.

Don't add anything but a small amount of oil to the system unless you flush everything. Otherwise you can easily get it over filled because you don't know how much oil is in the system to start with.

As far as rates for this work in your area, I'm not much help. To fill with UV dye, an ounce of oil and some R12, I would think you would be looking at somewhere between $100 and $200.

Good luck,