How to make shader that uses vertex colors to colorize mesh but accepts shadows?

Hello, for my game I'm using artstyle that resembles early 3D games (flatshaded, untextured, lowpoly, lowres 3d). For now all levels are built using 3D primitives shaded with Diffuse shader with just color.

However, I plan to retire the current system in favor of SabreCSG which recently went free and open source. The problem however is that the only way it colorizes walls is via vertex colors, not by changing color value of material of that side (and making new instance if necessary). There is a vertex color shader included in the package that colorizes walls, however it is unlit.

How to add shadows (both shading and shadows from shadowcasters) to it? I want also to avoid PBR pipeline for the shader both for performance and artstyle reasons.