I am putting new a/c compressor and it came full of oil. Do I still need to vacuum system or can I just fill it?
Always vacuum the system (or even better have it vacuumed by a professional with the proper, expensive equipment). Non condensible gases prevent the expansion device from doing its work and raise head pressure. Also, the oil, if POE or single end capped PAG, is hygroscopic and can be degraded by the air's water content, turning it into acids that slowly eat away the metal inside the AC loop. Also, not vacuuming the system prevents the refrigerant charge from being loaded by weight as liquid in the high side with the compressor inoperative and the engine off, the least harmful charging method for any compressor and the mandatory one for variable displacement compressors. Fit the new accumulator/dryer minutes, or even better seconds, before the system is vacuumed (or have it fitted).
Also, it's a wise idea to properly dispose of any oil found inside the compressor, that might well be assembly oil and not refrigerant oil, and replace it with fresh oil of the correct type and viscosity for that compressor (as an example, modern variable angle swash plate compressors such as the Delphi CVC or Denso 7SEU17C always take PAG 46; as for its quantity, the same quantity of oil that came out of the old compressor if it didn't seize, or the entire system oil quantity if the compressor seized and/or the system is flushed; also, ensure to rotate the compressor's shaft by hand 10 times, by rotating the pulley if clutchless or by rotating the clutch hub if the compressor is equipped with a clutch, right after the compressor is fitted on the car and before the engine can rotate it).