How does Directon Light work in "LightMode=Vertex" and Rendering Path is VertexLit

it is weird,the default vertexlit material can work,except my own shader which just caculate the direction light in vertex shader when lightmode=Vertex and camera’s rendering path set to VertexLit,it is black,anybody kown why?

But when in editor,it seems works,the attached pic is the screen capture of Game view