Re: Paris, Rome, Venice
Venice has not much to do? 
if it is the type of person who will spend 2 days in vatican museums, then there is a lot to see in venice. There are 2 dozen or so museums, which cover all aspects of venetian history. and venice at one time was a major force in the world.
if it is someone who just wants to get a flavour, then vatican is at best a half day outingm and venice an overnight trip.