While I appreciate the answer, it's not a great explanation of the balloon example, since a balloon starts out effectively containing nothing. The problem there is that as you add helium, the volume of the balloon increases.
I'm a little more comfortable with saying we do some weighings in a vacuum (say, to determine the density of air at a given pressure) than with saying we'll start with a cylinder that contains a vacuum. For example, our 40kg cylinder, when airtight and containing vacuum, will weigh less than a 40kg object should, and I think that muddles the example.
Only the difference between vacuum cylinder and helium cylinder matters. (Since the buoyancy in air only depends on the volume of the cylinder, not what's inside.)
I'm a little more comfortable with saying we do some weighings in a vacuum (say, to determine the density of air at a given pressure) than with saying we'll start with a cylinder that contains a vacuum. For example, our 40kg cylinder, when airtight and containing vacuum, will weigh less than a 40kg object should, and I think that muddles the example.