Try cleaning the air filter screen and the airbox.
Also, I don't know if vespa carbs have metering rods, but if you pull the top end of the carb off and there's a needle sticking out of the bottom of the throttle slide, take that out and move the retaining clip on it up one notch and put it back together. That should keep it from getting too much gas. That's about all I can think of if everything else is clean in the carb.
Oh, also check for vaccum leaks between the carb and engine. To do this, start the bike up and while it's idling, spray some ether or starting fluid (carb cleaner will work, too) around on the connection between the carb and engine. If the engine speeds up or slows down without touching the throttle, then there's a vaccum leak. Pull the carb off and put some gasket sealer on the manifold.