Both the Menger Sponge and the Vicsek Fractal in 3D can be constructed by starting with a cube, dividing it into 27 smaller cubes (3x3x3 grid), removing some of these cubes, and then applying the process recursively for each smaller cube.
In the case of Menger Sponge, 20 cubes are left and 7 are removed: the central cube and the cubes at the centers of each of the 6 faces. In the case of 3D Vicsek fractal, the 7 cubes forming a kind of 3D cross are left and the other 20 removed. So in a single iteration, the cubes removed in one fractal are those which are left in the other. One could say that the unit cell of one fractal is the complement of the cube and the unit cell of the other fractal.
Is there a name for this kind of relation between two fractals? Of course, the whole fractals are not complements of each other, only their unit cells are.