If you have a <1mm thick layer of vacuum inside an armour plate, be it for vehicles or body armour, wouldn't it stop explosive shockwaves from penetrating through the armour? This is, of course, presuming the explosion is not powerful enough to crumple the plate and either force it to make contact with the back plate, or simply rupture it and ruin the vacuum.
Strength of Steel
Flexure Strength of 2mm of mild steel: 381,870,000N/m^2. According to one answer, that is almost four thousand times the force of 1 atmosphere of pressure, and over twice the force of 1kg of TnT at 1m distance. Unless there's some weird mistake with that source, I don't understand why people are insisting, without calculations or sources, that vacuum is impossible to contain without a huge amount of steel, when glass jars can do it.
I have added the hard science tag, in the hopes someone will actually demonstrate whether several mm of steel can't withstand a vacuum, despite glass jars being able to.
However, please note I never limited the material containing the vacuum to steel... I just mentioned it as an example.
Back to the Question
I was thinking of arming soldiers with this sort of vacuum plate, due to the prevalence of explosive-based weapons in the setting.
Notably, shockwaves can still travel through the joints of the armour, since you can't have it totally free floating:
However, that should reduce the effectiveness of the explosion, if it is forced to go through narrow chokepoints? You can also focus padding on these areas to dissipate it.
The joint areas might be at the shoulders and hips, arm and leg joints, protecting the lungs, heart, and other organs from direct shockwaves. Helmet a bit like an astronaut or deep sea diver, with a vacuum gap between the visor's glass panels. Neck will have to make do with a vacuum gorget and plenty of padding, unless you want to make it rigid like with tournament armour.
So, would this work? I don't mind some magic to make it work, but I'm interested to know why this hasn't been tried.