Also, vacuum is a bit misleading, since there is no known place in the universe which is a perfect vacuum. There's always a particle per cubic km or something, even in deep intergalactic space.
Well yeah, but vacumm flasks work because we create a near vacuum, which greatly increases its insultion properties.

Whenever I say vacuum, please think the word "near" before it.

Ok so vacuums leak and slowly regain neutral pressure, that's a heavy flaw. Why not fill the cavity with argon then? That's still a much better insulator than a single wall, right?