On Saturday, November 2, 2019 at 6:56:25 PM UTC-7, pyotr filipivich wrote:

The Mythbuster guys managed to collapse a tank car. But they had
to "ding" the tank to compromise the structural integrity first

Yeah, that's the key; a round tank (or even a flat lid atop a can) doesn't
spontaneously break symmetry, you have to START that. So,
an undented drum should hold vacuum well (it's only 14 psi)
just as a beer can should be hard to crush. But, I can
hold the beer can and dent the sides with my fingers, then a little
twist... and it's flat in seconds.

Half-crumpled 50 gal tanks can be re-formed to original shape, quickly,
if you have a blasting cap handy...