So it's not really exactly the vector normal, but something that weights the shading across a triangle. Blech.Associating the normals with the vertices allows the renderer to 'shade' the triangle based on the different normals at its 'edges' (the vertices). Its how our low poly meshes look smooth.
So if you are actually interested in rendering a distinct, sharp corner, you need to have duplicate vertices at the corner, with one set of vertices associated with one surface pointing normal to that surface, and a second set of vertices with a different normal direction applied to a different surface.