Where to fly out of in Italy

It could be a toss up but I would opt for Florence. If returning to Canada, that flight will most likely will be a connect flight with an early if not very early departure. Departing from Rome could give a more reasonable departure time.

If you are planning on actually staying in Venice, I could see it being tricky getting out of early in the morning, that said, I know nothing of Amerigo Vespuci. We flew into Venice, and it was a breeze, really, and pretty fun. If it's in the morning you can take a water taxi or get on an Alilaguna and be taken directly to the airport. If you are rushed, I would take the taxi, they can come directly to you if you are on a canal. If you aren't rushed and there aren't enough of you to warrant 90-110 euros (Alilaguna is ~12-15 eu pp), I suggest the Alilaguna. It's not as fast, and has preset stops but is still a great way to get out of Venice.
If you opt for taxi, I've been told you can save 10 euro booking in advance, FYI

Where will you flying to? If you are flying back to Paris to pick up your flight back to Canada., then either airport will work fine. There is public transportation to both. However, if you are expecting non-stop flights back to Canads, I'm fairly certain that you won't find any out of Florence and I don't think there are many from Marco Polo. Amerigo Vespucci airport is a smaller airport. We've flown in there from Frankfurt on Lufthansa. You won't see planes much larger than the Boeing 737 or small Airbuses flying in and out of there. If you fly out of Marco Polo, watch out for the really good airfares that have you departing early in the morning. The earliest public transportation to Marco Polo on the buses from the Piazzale Roma begin at 5:00am. It takes about 20 minutes to get to the airport but if your flight departs at 6:15am, - you get the idea. However, there is always the priviate water taxi that will pick you up as early as you need and whisk you to the airport. Cost is 90e-100e for up to about 6 people. It's a great ride if you need it.

I'll repeat heer what I posted on your other thread, as a follow-up to larry's comments: "Also consider the time of the flight, especially if you are flying all the way back on the same day (no stopover in another city). It seems that the Venice flights leave early in the morning ( often before 7). We see lots of questions here on how to get to the airport at that time of the morning. It might be even more difficult with kids." In other words, beware of those early-morning departures from Venice. The water-taxi may be fun, but it is expensive, and I wouldn't want to get my own kids up that early in the morning to make a flight that could have been avoided with different planning.