in the vertexshader you have to define vecdiffuse:
VertexShaderConstant[18]=<vecDiffuse>;

you can output vecdiffuse to oD1 in the vertexshader:
mov oD1,c18

and access it with v1 in the pixelshader:
mul_x2 r1,r1,v1 // modulate light intensity against vecdiffuse