Itinerary Help, Rome Hotel

I think I have finally figured out an itinerary for our Italy portion of our trip. We don't have a lot of time, because my husband has to work and we are including a cruise kind of in the middle of things. I am traveling with my 18 yr old daughter (her first trip) and my 73 year old parents (they've been to many of the places we are visiting, but aren't very mobile, my mom needs to sit down every so often when she walks)and of course my husband. We are going to spend a couple of days in Rome (mainly for my daughter's benefit), then heading to Sorrento. We will spend three days/two nights there, then head to Siena and spend two to three nights there. From there we head to Venice to pick up a boat that cruises to Dubrovnik and Croatia. After that, we will only have two days to do something. My husband wants to see Switzerland in the worst way, but I think we will have to "settle" for Lake Como due to time constraints, unless people think we can just do a train ride and then hop on a plane. Our main dilemma is car vs. train. My mother's limited mobility is swaying us more towards renting a car (probably minivan) to do Siena for sure, but my husband wants to pick up the car out of Rome, then drop it off in Venice. If this is the case, where should we stay in Rome (does it matter?) We primarily want to see the Vatican/Sistine chapel, then maybe the normal tourist sites (Coliseum, etc). Any advice would be greatly appreciated.