I'm a vespa bravo owner myself which I recently purchased from my sister. At first it didn't start well but after fiddling with the carburetor and air box, it now runs fine.
At some time in my fiddling process, I completely took the motor out of the frame and after reassembly my lights wouldn't work anymore.
Symptoms: front light only working when using my brakes, and even then dimly. Horn just barely audible.
Solution: After almost seizing my brain from looking at the electrical diagram, I understood that during reassembly, I had connected the red and the black wire to the wrong connector on my motor, red to black and vice versa.
I don't know if your problem could be as simple as that, but to me it sure sounded a lot like my problem.
My problems are solved now, apart from the leaking carburetor that I refuse to open up once more and the front stationary light which blew not 1 mile after installation...