Vacuum deoxidation is a method which involves using a vacuum to remove impurities.  A portion of the carbon and oxygen in steel will react, forming carbon monoxide.  CO gas will float up to the top of the liquid steel and be removed by a vacuum system.
As the chemical reaction involved in vacuum deoxidation is:
the reaction between carbon and oxygen is represented by the following chemical equilibrium equation:
where P is the partial pressure of the carbon monoxide formed.
Decreasing the oxygen activity(a) will result in a higher equilibrium constant, thus more product, CO.  To achieve this, subjecting the pool of steel to vacuum treatment decreases the value of P, allowing for more CO gas to be produced.