Being stuck with a Sluggish Vespa myself, I started the research and tinkering and here's what I found. The Vespa Caio's came in 3 different speed rated models from the factory, a 20mph model (the one I unknowingly bought), the 25 model , and a 30 mph model. After talking to the techs the difference between the peds is in 3 different areas. The Exhaust, The Air Restrictor attached to the Carb, and the Jet in the Carb.
After tweaking the Exhuast on mine; cutting it open gutting it and welding it back together again; My top end went from 20 to 24 mph. Next I changed the timing; this is done by changing the gap on the points. Factory calls for .016 setting, I gapped mine at .020. The wider gap advanced the timing and improved performance; again my top end increased to about 26-27 mph.
Next the big step, Tuesday my new jets will come in, it was a pain finding a supplier but I found one. The stock jet is a .043 and requires the air restrictor to be attached for the moped to run properly (Limiting the air and fuel the ped gets so limiting the power it can acheive). The jet I ordered a .049 does not need the air restrictor. With the new Jet and the Air Restrictor removed (the small plastic box attached to the carb) I should get about 35+ mph with no problem.