Before we worry about a vacuum, we can operate the magrid on air. If you look at the transcript for “Should Google go Nuclear” , on page 10 Bussard says of the WB2: “All these tests were run on air or argon”, accompanied by this picture:
I think this means forming an electron cloud in the center of the magrid, despite presence of air. This sounds like a Non-neutral_plasma . I suppose you would operate the magrid normally… bring the shell to a high potential, turn on the electron beams, bring the magrid up. The streaming electrons ionize the air. The air ions rush to the center. Some of that energy is released as light. Even if it’s possible to burn air, it may take low pressure air, requiring a vacuum vessel and pumps.