If the refrigerant compressor needs to be replaced, do I have to use the vacuum pump, or just replace the compressor and fill up with refrigerant?

While the compressor is being replaced, the system will be open to the atmosphere. Pulling a deep vacuum after the repair is completed will remove air and moisture that has collected in the system. This is an important step to maximize system performance and reliability.

When a system is opened for repair, air and moisture will enter the open components. It is important to remove this air and moisture with a thorough evacuation before charging.