[cwillu]

You can support two panes with a gas in between with a strip of soft rubber, whereas with vacuum, I suspect the rigidity of the frame needed to support the panes against atmospheric pressure can easily kill the gains you make in isolation.

[stephen_g]

There are vacuum insulated units [1], they need tiny plastic 'microspacers' in them to keep the glass layers apart.

1. https://www.pilkington.com/en/global/products/product-catego...

[cwillu]

And those spacers are rigid, which will remove a lot of the acoustic gains you'd otherwise get from using a vacuum.

It's worth noting that, although the page you linked says “good acoustic performance”, the vacuum windows they offer aren't mentioned on their noise control product page, nor listed in their sound simulator tool.