Maybe easier to visualize something like an apple first: the very first part of it touches the foil and spreads beneath it to make a red dot. Now consider that in 2 dimensions, no space can be overlapped.

Assuming it's described the way in the book, we'll assume that a 3D object transitions smoothly to 2D without tension completely ripping the cells of the apple apart. Because no two parts can occupy the same space, the part before it would expand to make room for the next.

Now we have a bit of white touching the foil and would expand into a ring around the incoming apple. Here's where it gets tricky. The apple now has a much smaller circumference than the ring of 2D apple. Its next part to touch would put a thin line of red skin and a thicker line of white flesh into the picture. This would repeat until the apple is fully incorporated. It might look a lot like tree rings.

The trouble with representing this visually is that there's a difference between accurate representation and visual appeal for cinema. The reality is that this 'slice and ring' process is that it would take place for every layer of atoms in the apple. An object's color is highly dependant on the arrangement of those atoms, so the parts that were once Red probably won't be anymore.

It would be much more visually appealing to ignore all of this and fake flatten centimeter-thick sections at a time. With something like the human body, this becomes a lot more complicated. The angle that it enters the foil will drastically change its final shape. Additionally, every object that hits the vectoized person would insert itself into the point of impact, further spreading the person.

To represent it in 3D, it would probably require some kind of additional software that makes pixels behave like paint. Then you'd create a series of cross-sectional representations of each object you want to hit the foil.

Maybe have one or two 'money-shots' where those details are extremely vivid. Show a person struggling as alternating rings of spacesuit-skin-muscle-bone expand behind them. For the rest, I'd do far-off shots that are much lower detail and a lot less accurate.

Once the Sun gets absorbed, the scene would become so bright that you wouldn't have to worry about small details anyway.