Ok, say you have an extremely lightweight yet strong material that you could make a large vacuum chamber out of. It would be quite effective as a lifting device.

And as you note, regulating the pressure inside with bladders would enable you to adjust the buoyancy, this is actually a system that the modern Zeppelin NT uses, though not for adjusting lift in response to loading/unloading, more as a general setting for the area and air pressure.

The NT is usually calibrated to weigh about 300kg and uses movable propellers to move up/down and relies on the airfoil shape of the inflated envelope to generate the tiny amount of lift to stay up while moving forward.