All this talk about black holes goes right over my head. Aren't black holes an hypothetical construct offered by astronomers who can't or won't accept the implications of plasma physics on a galactic scale?
Anyway,
Magdeburg hemispheres - Wikipedia, the free encyclopedia. Vacuum bubbles don't scale.
OTOH pure steam has approx. the same lift as helium.
The Flying Kettle Project - balloons and airships filled with steam!
EDIT: Here. This is an icosahedral cage around a tensile bag, stretched to a 3.2 fineness ratio.
A more developed design would have a subdivided shell with a low-stressed, insulative skin. The inner skin would be highly tensed and made from some unobtanium that would have to be impervious to the corrosive effects of water on one side and the embrittlement effects of hydrogen on the other.
The upside is that instead of all the insulation being in a thin outer skin, the heat is inserted into the hydrogen in the inner bag and the steam become a layer of insulation as well. The steam would be at overpressure; so, worst case, the hydrogen might be at ambient pressure. Deeper vacuum = more lift.
Condensing the steam kills lift to facilitate ground handling. If it is designed to ever land.