[i5heu]

There is no light and strong enough Material that can keep a vacuum in the size needed for a Zeppelin on earth so it could fly.

On mars this could work.

The brain confusion that a room of vacuum is lighter then air and will therefore have a lift is amusing.

[jfim]

I think you misinterpreted what the parent is saying.

He's saying that a bag that contains only vacuum would be the lightest, but as it contains nothing, it would just collapse into having no volume due to the atmospheric pressure. That would obviously provide no lift. As such, you need something in the bag that will fill the space.