Put the buffer and a magnetic stirrer in a vaccum flask (thick glass, a small connection on the side and a large opening on top), maybe up to a third of the flask, and start the stirring. Put a stopper on the top of the flask. Connect to the vaccuum pump, it will start degassing. This also works with one of these small void trumps you connect to the tap. Best degassing is obtained if the stirring is not too fast, and we used to tap the flask (with our hands) to watch the bubbles forming and accelerate degassing.