I'm currently reading Diamond Dogs, Turquoise Days by Alastair Reynolds. In the second book, there's a water-world where there are airships, and a whole city held aloft in the air by "vacuum bladders".

I'll admit when I first read that detail in a description of the blimp-like airship the 2 characters are riding right at the beginning of the book, it took me a second to realize exactly what it meant. My understanding of airships like blimps and such hinges on something being in the gas bladder... But yes, nothing is certainly lighter than something! I haven't yet encountered any technical description of how the bladders are constructed, which I look forward to reading.

If you can make a vacuum container lighter than air then go for it.