348 R12 refrigerant Oil Charge question

That sounds like pretty standard procedure to me, others may answer different but;

There wouldn't be a need to vac out the system unless you were doing a flush to convert to 134a or discharging it to take lines apart and change parts like compressor. It just needed a top off so that's what he did for you.
The gas leaks out much easier then oil so unless you saw some oil or think you lost some how it should still have plenty.

 

The right way is to evac the system, hold a vacuum on it, add freon, know the proper gauge pressures as finishing targets. That said as a home hack I would do it right but not do it right if I opened the schrader and plenty of freon seems to be leaking out implying that there is no moisture or air in the system from the last tine I properly sucked out the system and add freon directly in using gauges for the right pressures. Gauge pressures are important or knowing the fill weight of an empty system and weighing the freon charge to make sure you got the right amount. You can also be even more dirty and add freon and look for your temp split from ambient and call it filled.