Vue materials are applied to Vue objects, C4D materials are applied to C4D objects, so Vue trees wouldn't be cell shaded. 
What you get with the xStream plug in is correct lighting on all elements based on the Vue atmosphere, Vue elements reflecting C4 elements and vice versa, Vue elements shadowing C4D elements and vice versa, C4D objects projecting shadows according to the Vue sun.
If you want cell shaded Vue trees, you need to export them form Vue6, import them in C4D and apply C4D materials to this new object.
I suggest you read what exactly Vue xStream is at e-on's site.