are your carbs identical? (both 12.10? both are 12.12? both are 12.7?)
are your gearboxes identical ratios? (eg are both 11:1? 8.5:1? other?)
are your rear pulleys identical? (eg are the both 80mm? 90mm?)
are your front pulleys identical (you will prob have to pull the belts and measure the inner "race" belt pulley surface diameter to tell because not all front clutch bells have the pulley diameter cast into the surface)
identical stock pipes? (all stock vespa pipes btw are "buzz" killers ...pun intended... but some are really small headers or short chambers compare to others)
it is very possible for two different single speed vespas to get the same top speed but have noticeably more or less torque than the other.
For example, if the ciao is on a 12.12 carb with hiflow airbox and a tall rear pulley 90mm or 100mm and it has pretty high transmission ratio like 14:1 or some such, it will certainly pull very hard off the line but ultimately max out its top speed at around 25ish because the gearing is so high
On the other hand your bravo has 16" rims which is technically a speed limiter and though it could be on nice fast transmission ratio like 8.5:1 or even 7:1 but because its got a small 12.7 or 12.10 carb and small stock pipe, it ends up being a dog off the line and eventually revs out enough on the small carb to get that rear end spinning finally at 25mph-ish before its just cant breath any deeper
All that said, lets pretend you are literally identical setups with the exact same carb and airbox and jetting and exact same pipe capacities and exact same front and rear pulleys races mm and exact same transmission ratios.... the issue can still be typical stuff like worn rings, leaky decomps or nasty head or base gaskets, carbon filled exhaust manifolds or pipes or very very often, different intake spigot "intake patch" boring inside the cases rotary valve surface (the factory shipped some really tiny ones as well as healthy sized ones but then slapped on everything else the same so you dont know it until you get a look inside the spigot at the rectangular valve port outline, or when you split the case and look at the patch from the inside.