Here is the coverage map (very well hidden!):
http://www.areaaziende.vodafone.it/1...ty_key=az_must
AFAIK there's no shop list on Vodafone website. There should be one at Termini train station as well as at Fiumicino airport. Usually Vodafone shops are always very well placed at strategic places.
I think you should be able to activate English prompts, but I honestly don't know how... I have a Vodafone sim-card, but it's a business postpaid, it's totally different, so I cannot help you with Vodafone prepaid.