To people just starting to learn about scooters, it's difficult to tell just what's what- between brands, and even between different models within the brands, it's kind of difficult to tell what's what. This should serve as a crash course, although the two scooters I'm comparing are different enough to begin with that they'd very rarely be confused.
Vespa has been making scooters sincethe late 40s. When they started to produce scooters, the design was elegant, and has been continually refined through the years. Some differences aren't readily noticeable, such as engine reworkings. Locations of gloveboxes and headlight are noticeable, but sometimes hard to keep track of. For purposes of this comparison, we'll make it fairly easy- a 1964 largeframe scooter vs. a 1979 P-series. (In 1966, Vespa started to produce a slimmer, more compact version of their scooter- this is known as the smallframe. The Smallframe, largeframe, and P-series make up the majority of the Vespa body styles.) The first readily apparent feature of the P-series scooter is that it's boxier than the original series. Where the original scooter had very curvy and graceful cowls (The two 'bubbles' on the sides of the rear of the scooter) the P-series has a roughly defined angle across them. Unfortunately, the following picture doesn't have a clear picture of the largeframe's cowls, but if you look elsewhere it should be fairly obvious. Another difference is that the P-series is actually much larger than the Large-frame. This is mainly due in part to the 10" wheels that are used, as opposed to the 8" wheels on the earlier models. Yes, the 8" wheels are very small, and make the rider feel as if they're skimming on the pavement. The p-series also has a better suspension system, which along with the bigger tires, allows for a much smoother ride. On the front of the legshield, there are several more differences- in a nod to the DOT and standards for roadworthiness, the P-series scooters have turn signals- the vintage scooter rider has to rely on old-fashioned hand signals. In the early 70's, Vespa moved away from the flowing script badge that had graced the front of its product, and to a simpler block-letter affair. Also, the horn was now surrounded in a big hunk of plastic, as opposed to being a small metal horn that mounted right into the body.
Other difference not apparent in the pictures-