VIPs are shaped like thick rectangles (fairly-flat boxes, cuboids), for example 20 mm x 400 x 600 mm. If manufacturing started with a sheet of styrofoam - a thick rectangle - I would not be surprised that the VIP has the same shape. In videos I've seen* of vacuum-packing loose powders, the result is not panel-shaped. (Fumed silica is not technically a powder, but for the purposes of this question I think I can say it's powder-like.)

I know the vacuum-sealing makes it retain "a" shape, but how does it attain the panel shape?

Do the vacuum chambers include flat plates that physically shape the powder inside their envelope before/during the vacuuming? I've seen many types of vacuum chambers, but none include plates for shaping the contents. How does fumed silica attain a panel shape when used in vacuum insulated panels?

* Video of powder being vacuum-sealed in a vacuum chamber, not resulting in a panel-shaped thick-rectangle: youtube.com/watch?v=2eES_QB1bOM?t=90. (The same process is repeated three times. Just see the first - they're all the same.)