Return from VCE to LAX via FCO

Alitalia is part of the group that includes Delta, KLM, and a number of other carriers. If they sell you a ticket with a too short layover, they have to rebook you, and it seems there should be lots of options. The only question I would have is about security. I'm remembering that passport control for U.S. destined flights happens in T5, after which they shuttle you over to your departure terminal. That was for a trip originating at FCO, however. For what it's worth, my instinct would be to go with the itinerary you described.

Are you absolutely certain that the FCO flight at 10.10am is direct flight back to the US. That is a bit early for direct flights. I have never connected in FCO for a return flight to the US but I am sure you will stay behind security from Venice. That should make the connect easy.

Both flights are operated by Alitalia (Delta and KLM, Alitalia's partners, are in code share). The AZ620 FCO-LAX is a non stop. Your departure time of 10:10 from FCO is correct. I've taken that flight both directly from Rome or even flying from Florence first. The flight departs from T3. Not sure which terminal your flight from Venice will arrive at. Regardless you will have to go through security again and passport control in Rome. Your luggage will go straight to your LAX flight and you won't have to pick it up, so don't worry about checked baggage. It's a tight connection because the LAX flight will start pre boarding at 9:25am so you have to rush, but you should be ok. Nevertheless even if your VCE flight is greatly delayed and you can't make the flight LAX flight, Alitalia will rebook you for the next day (and cover your hotel) or maybe reroute you to LAX with a flight via ATL or JFK, both of which depart later, (with either Delta or Alitalia, as Delta is a partner).

However I seriously question your premise of starting from Rome first. I would never do that to myself if given a choice. I would definitely do the exact opposite. I would fly to VCE first, even if that means a layover somewhere first (JFK, Paris, Amsterdam, or Rome). I would then return from Rome with the non stop AZ620. Getting to the Venice airport for a 7:40am flight will be a royal pain in the arse since it will require you to get up in the middle of the night to catch a boat for a long and expensive journey to the airport, which is located in the mainland. Lots of people give up their last night in Venice to sleep near the Venice airport for that reason. Instead if you do it my way, and spend the last part of your trip in Rome, you can comfortably take a taxi from downtown Rome at 7:30am and be at the terminal 3 of the Rome airport 25-30 min later, with more than enough time to make your 10:10 non stop flight to LAX. In addition I'd rather visit Venice, at the start of my trip, when I'm still jet lagged, than Rome. No thank you. You might be run over by a bus in Rome (after being pickpocketed by a gypsy kid) while you are walking half asleep like a zombie for the first two days of jet lag. The worst that can happen to you in Venice is accidentally fall in a canal, which is very unlikely, but would have the unintended benefit of fully waking you up.

That is smart traveling. Believe someone who has flown from Italy to California about 40 times (each way).

Having been on the RS tour to Venice and Rome both, I'm not sure I would have a preference which way to do it, whether starting or ending in Rome. For your trip the way you're currently planning it, I think that layover is too tight. The FCO airport is a cluster. The good news is that you'll arriving and departing from one of the "main" terminals (I believe T5 is reserved for US carriers flying to US cities, as well as a few others), which should make your life easier, but I would just try to book something with a longer layover. Is there perhaps an earlier Alitalia flight from VCE to FCO? They usually fly those short routes several times a day. I just hate having to rush to make a connection.

I suggest you heed Roberto's advice, or at a minimum travel to Rome by train the day before your flight, and spend you last night there. I have seen so many questions on travel forums ( especially Tripadvisor) about difficulties getting to the Venice airport for an early morning flight. It is not easy and causes a lot of stress ( unless you want to splurge on a water taxi).

If you are already in Rome the night before, you will not have to worry about missing the connection to the LAX flight.

my daughter and I on a rick steves tour, ending in venice. We treated ourselves and hired a water taxi to the airport very early hours!
St marks square was empty, We felt like we have a private water cancel tour to the airport.
On flights I took a Alitalia flight from KLM Amsterdam and they lost my luggage for the start of my tour. :(
Just a few comments.

my daughter and I are headed out in Sept again for the Village tour, excited! Have a fun tour

I don't understand what T5 (terminal 5) has anything to do with anything. T5 is not a terminal, it's just a check in processing center. No gates are present at T5 and no airplanes arrive or depart from there. Even if the poster had to connect to a US carrier, like Delta, the Delta flight is still departing from T3. T5 would be used only by the passengers departing from Rome (not connecting) for check in and security. After those passengers are done with the check in and have gone through the metal detectors to the sterile area, they are bussed airside to T3, where the actual gates (and the airplanes) are.