The vaccum pump will not remove any oil from the system. The existing vacuum can be used to pull in any additional oil that you may need to put into the system. Adding oil to an existing system is a dead reckoning situation. If the compressor was running normal with no noises 1 or 2 ounces would be OK. If the leak area had no visible oil traces, it may not need any. If the leak was on the low side, there may not be any oil visible. The high pressure side will usually leak oil along with the refigerant. Make sure the oil is compatible with the refrigerant. If the system was at atmosphere, a new dryer is a must. The oil could then be added to the dryer.