Just as the title says. If you pumped all the air from a balloon and somehow (handwave) kept it from collapsing. Would that balloon produce more lift than a hydrogen filled balloon? How much better would it perform?
Not very much. 1 mole of a gas occupies 22.7 litres at 0°C. One mole is the molecular mass in grams. Thus one mole of hydrogen weighs 2 grams, and one mole of air is (28 * 0.8 + 32 * 0.2) = 28.8 grams. Therefore a hydrogen blimp gives 26.8 grams/22.7litres lift (1.18 grams/litre) and a vacuum blimp gives 28.8grams/22.7 litres (1.27 grams/litre), a difference of 88 milligrams/litre.