Not likely if they’re vacuum.

Yes likely if they're rigid, which is the whole point of vacuum balloons. The trick is coming up with a material that is strong enough to withstand the atmospheric overpressure trying to crush it, while also being sufficiently lightweight.

carbon nanotubes, the answer is always carbon nanotubes ;)

Like some foamy spheres. I couldn't understand if foamy spheres with a plastic film around them or just foamy spheres with self-contained holes.

Oh, and on the color of aerogels.