As a former rider myself, I wouldn't recommend it. But if you insist, there are Vespa rental places all over Rome. They'll even deliver one to your hotel. You need a drivers license from your country, plus an international drivers license. In the USA an international driver license takes about 20 minutes and costs about $15. How much it costs depends on the time of the year, but it's not much. They provide helmets. You are responsible for any damage to yourself and others. You'll need to leave a deposit on your credit card, the size of which depends on the size of the bike, 500 euro deposit for a 50cc one, up to 1000 for a 250. While emergency health care in Italy is free, shipping you back to the USA is not, so check your insurance.