Considering the limited power of the Vespa's engine the added weight of the canon I can’t imagine that the Vespa would be able to hit 35 mph.
Ah a 50cc Vespa couldn’t make 35 mph without the cannon
At least not the one I used to have
You’d get thrown head over heels if you tried to fire the thing while in motion. Your forward momentum would continue while the Vespa would likely go backwards for a bit, lol.