1
If there is no geometry shader, then the vertex shader must at least do the projection transformation because this is the space the hardware expects the vertices to be in when leaving the vertex shader (if there is no geometry shader). If there is a geometry
shader, the job of projection can be deferred to the geometry shader.
2
A vertex shader (or geometry shader) does not do the perspective divide; it justdoes the projection matrix part. The perspective divide will be done later by the hardware.