Italy airport timing

Is booking a flight to Venice from your origin really that much more than cobbling together a separate flight from Rome.

I have not connected thru Rome. I did fly in/out to Greece and the plane we took was parked out away from the terminal, necessitating a bus ride to the plane. This means boarding happened earlier than for a plane parked at the terminal.

A 1 hr 45 min transfer on separate tickets is not something I would attempt.

If a single ticket routing to Venice is not possible from your origin , I have a couple of thoughts.
1) Allow more time between flights
2) The train from Rome to Venice is 4 hrs.( no changes) Cost for walk up fare is $100-$120

If your final destination is Venice, why not book a flight from Lisbon to Venice instead of taking 2 flights.
Did you already purchased the LIS-ROM ticket?

Once you go through passport control in Lisbon you won't need to do it again in the Schengen region.

For now there is only one direct Trenitalia train from FCO airport to Venice, it departs at 13:53 and arrives at 18:38. A walk-up ticket costs $ 127 pp (€ 106). I wouldn't run the risk of buying a discounted (but not changeable) ticket, flights can be delayed for hours.

You could save some money taking the FL1 train (€ 8) from FCO to Roma Tiburtina station. At Tiburtina get on the first high speed train train departing to Venice, choose between Trenitalia.com (€ 92) and Italotreno.it (€ 88) , depending on which train leaves first.

thanks all! Trying to save some time with a flight since they cost the same ;)
The flight to Rome was dirt cheap and is purchased and locked in.

the time frame might be pushing it though!

If time is what you're trying to save, don't forget to add in the extra time you'll need to get from the Venice airport to the centro storico. The train, on the other hands, takes you right to the centro storico.