There are several models of "Vespa" but I think you just mean scooters in general. Most 50cc models are speed limited to about 60km/h (or about 40mph for you yanks).  Honda, Yamaha, Vespa, they all make decent scooters. Check your local laws as to what license is required, as he may have to take the test before he would be legal to ride. As long as he wears a helmet, coat, long pants, boots, and gloves, he's protected from falling down. The real danger all us motorcyclists face is the other vehicles and drivers. I cannot stress enough that new riders should take a safety and riding course. It will teach how to ride defensively, stay aware, and not take risks in traffic.  Most local shops will have a poster up or be able to point you to a local course. In my case, at 16, wanted my grandfather's Honda Nighthawk 750, and the deal was I could buy it IF I took the safety course first. I had been riding off road for years, but the course taught me the skills I needed on the road. It was an excellent introduction, and kept me alive to keep riding!I made my new wife take the course before she could ride on the road, and it makes me feel much better when we go riding together.

the 250 will get up to 76 mph
the 150 about 60
the 100 about 40

I work for a Vespa dealership from time to time. Here is how it works:
Naturally it depends on the size of the engine. The Vespa engines that are out currently are 50cc, 150, 200, 250 (a few larger ones on the Piaggio branded scooters).

50cc-30-35
150-60-65
200-65-70
250-85-90

The 250 is fuel injected and packs a good punch. They are all very quick, meaning they accelerate up to speed quickly. In most states(countries) a motorcycle license is required to operate these. They are awesome and serious machines