The pressure inside a vial is the same as the air pressure around you. If you push in a pin and then pull out the oil, you are creative negative pressure in the vial (a vacuum) which makes it harder to pull out the oil. If you pull air into the pin, stab the vial, turn the vial upside down and then inject the air into the vial, you creative positive pressure in the vial. This pressre then helps push out the oil much more easily, and you never have the problem of negative pressure in the vial.
Think of those drinks cartons that you puncture with a straw to open...if you try to drink the whole thing without letting any air back in it's difficult. However, if you blow air into the carton, the pressure forces the liquid out into your mouth...........................