SCHISM does not use an explicit option to distinguish between a 2D model and a 3D model. Instead, it offers the flexibility to individually assign the number of layers at each horizontal node. The simplest way to make a vertical grid for a 2D model is to use the sigma coordinate on all grid nodes.

You can directly use this vgrid.in for any 2D model. It is a simple ASCII file, specifying a terrain-following grid with only a surface and a bottom (i.e., one vertical cell).