Did you get the Vector 40 Instruction Book, CLB-48? It should show you what you need to know. I haven't built a Vector 40, but suspect that a constant width bottom spar might be part of the plan for jigging the wing flat on the bench...might be! Steve
I built my Vector from a die cut kit many years ago,but I remember the spar piece was in the box cut, ready to go.I got it wrong , had to cut out of sheet.