My car is using about a qt of oil every 400 miles but not smoking. on 2009 Chevrolet Cobalt

The car is using oil but not smoking. I use it as a commuter and put about 800mi a week on it. There is oil in the air plenum where it connects to the valve cover. it stalls sometimes when I stop but runs fine on the highway. Still gets good mileage. The engine light is on but the codes say a vacuum error. I have checked all the hoses, and everything is tight What could this be?

Asked by for the 2009 Chevrolet Cobalt