Did you at all call setColor on the node, or something like that? If you do, a ColorAttrib with a flat color gets assigned (as you are noticing), which may be overriding the ColorAttrib.make_vertex() and causing the vertex colors to get overridden by the constant color.
getAttrib() by default returns the attribute set on that node; use getNetState().getAttrib() to get the effective ColorAttrib on that node including the effects of any ColorAttrib assigned to higher nodes.