If you're interested in the Vatican, try booking at the Cardinal Cecci.
It's a converted Cardinal's residence, the rooms are average, great breakfast buffet, the location is what makes it special. Many restaurants to walk to within a few blocks. If you don't mind walking, you can walk to Narvona, Spanish steps, Pantheon, Trevi, etc. If walking's not your thing, there are many city tour buses across from the hotel.
You can't hail a cab in Rome. You have to find one at a designated cab stand. That's difficult if you don't know where you are, or where they are. Don't try the subway. It's not real safe. Watch your wallet, the pickpockets are everywhere..