Vertex map painting tools are useful for a variety of functions. If you are working with game assets, you can apply vertex color maps (in addition to textures) to add variety and depth, and, if you are an animator, you can use Weight Maps to control how deformers influence deformations applied to geometry. The Vertex Map tools give you added control in either situation.
The Vertex Map tools are in the Painting tab under the Vertex Map Tools sub-tab of the toolbox. After you invoke any tool, Modo displays the tool's properties in the appropriate viewport panel. To paint, you need to select both a tool (a brush) and a tip (which designates how the brush itself adjusts the Vertex Map). You may need to select the target Vertex Map within the Lists viewport. If you use a mouse, drag the pointer over the surface to apply the stroke. When using a pressure-sensitive stylus and tablet, Modo drag the stylus over the tablet. You may also right-click and drag in the viewport to scale the size of the brush tip. For more information, see the Working with Vertex Maps topic.