Vertex painting is the process of assigning color values in one (or more) of the color-space channels to each vertex of a mesh. The uses of vertex painting range from permitting color/texture variations in your rendering engine without altering the UV’s or texture itself to creating masks for use in world space coordinate systems alterations (e.g., vertex position based on a wind and wind intensity input wherein vertices painted white will fully receive the input and vertices painted black will not receive the input and remain in fixed position).
Reference