If your idle jet is too big or too small, it can create a flat spot in the acceleration. Idle jets have two numbers. The first number is the size of the bore that allows fuel in, and the second is the size that allows air in. Available sizes are: 38/120, 42/160, 45/120, 45/140, 48/160, 50/120, 55/160. So a 45/120 jet would run richer than a 45/140, because the 120 allows less air to pass. To make things more complicated, some vintage Vespas came with plugged idle jets, which have no air hole. These are available in sizes 42 and 50.
Jetting is something that takes trial and error. See our blog of jetting instructions for more info.