Just into assembly of Vertex. Completed main box structure and have 2 Large Corners K8400-HQP-LC and 2 Medium Corners K8400-HQP-MC not used. Ten of each are shown in SUBKIT 15 A photograph in Manual Pt 1 but only eight are needed.
Have I missed something?
nopes 
Your Vertex 3D printer comes with all sorts of plastic parts that will not be needed. Follow the manual carefully, and take a look at the parts list at the beginning of each chapter to see what parts you will need for that chapter.