The geometry shader could be implemented as an extended vertex shader. The question is if it’s desirable to do so. One of the benefits of the vertex shader model over the geometry shader is that it’s 1:1 and thus predictable for the hardware and easily parallelized, whereas a geometry shader can output any number of triangles. Also, a vertex shader can benefit from a post-VS cache. A geometry shader implementing standard VS work would need to retransform the vertices for all triangles.