Maybe I can help. What we are doing is removing the air in the bag. This is a true vacuum. Were it in a solid vessel, the same amount of air would remain. Since the walls are flexible the bag conforms to the food inside, creating a surface that comes close the food. As I see it, that is the primary reason for removing the air. Otherwise, the air would cause the bag to float and insulate the food from the water. However, once you put it in the bath, the vapor pressure will increase, thus reducing the vacuum. If you want to further reduce the oxygen, you would need to purge the bag before creating the vacuum with an inert gas. By the way, I was a physical chemist.