I wouldn't get on a motorbike of any kind until I could already ride a bicycle. The balance works the same, but it's different.
A motor scooter has more stability than a bicycle, its not as wobbly or critical to balance. Plus a Vespa has a lower center of gravity, which also helps. So if you already can ride a bicycle, if you understand how the balance works, how to steer, etc., you won't have any trouble at all.
Plus Vespas and other scooters have automatic transmissions, so you don't have to learn how to use clutch and gearshift. You just turn the handle and it goes. It's designed to be really easy, and it is! Also a lot of fun!
If you've never been on a bicycle, trying to ride a motorbike will have a steep learning curve! 8^)