Flat Shader

I am trying to achieve this kind of art style: link

I found this tutorial: link and did what it said to achieve the look I wanted, but it splits the vertices and I end up with 5-6 times more vertices than the model originally had. This was not a problem in beginning, but now that I have to loop over vertices in code and do stuff to them, it is starting to be too much.

So I figured that I need flat shader to achieve what I want and still keep the vertex count low. Now the problem is that I can't find any shaders that do that, and now I'm not even sure if it is possible to achieve that with a shader since vertex can only have one normal. Can anyone point me in right direction?