Okay you are going to love this one. The Vespa Ignition is grounded through the brake light system. The field voltage for the condensor is grounded throught the brake handles, when you apply the brakes you disconnect the path and the field voltage then is grounded through the brake light to ground. However if the field is not initially grounded through the brake handles you will not get any spark. This and the confusing schematic had me stumped for an afternoon while I was chasing down spark. I finally found the problem, a broken wire in my brake wiring, but I also came up with a temp fix by making another ground for the field.
I cannot recall off the Top of my head what the wire color is comeing out of the Engine that goes to the breaklights, but I can describe it. You will have one wire going straight to your coil. Another that goes to ground. And the third that connects to two blue wires, one goes to the brake light, the other will go to the handle bars. Ground it and see if you get spark. Other than that check out your coil to make sure it works. Apply and disconnet power to it and see if you get spark with each cycle.