Connecting through Dulles domestic to international: 1 hr 40 minute layover

Yes. No issues.

It could work ok but there also could be issues. You might want to check how often your flight from Phoenix is on time or delayed. You also might check what later flights from IAD to Venice United has that day in case you need to be rebooked due to a delay. I would hesitate if yours is the last flight of the day. If you happen to have the bad luck to have to use the dreadful People Movers both arriving and departing Dulles, that could add considerable time. Because I live in DC, I use Dulles often butI never have had occasion to transfer through Dulles.

The issue is not whether a 1:40 minute layover is doable, but whether you will actually have 1:40. If your Phoenix to Dulles flight is delayed, then you risk missing your flight to Venice. And unless United has another flight later that day, you will be delayed at least a full day. Our flights from Florida to Europe usually require a connection in Atlanta or JFK on Delta or Dulles on United. On more than one occasion our flights have been delayed resulting in a stressful race through the airport to make the connection. For this reason I generally allow no less than 3 hours connection time. Good luck.

Personally, I would be ok with a 1h40m layover and would choose Dulles over Newark. If your flight arrives on time, this will be plenty of time to connect. Even if your flight is slightly delayed (under an hour), you should be ok. Newark has been having a lot of delays recently blamed on staffing shortages in air traffic control so it would not be my first choice.

Of course, no one can predict what will happen on the day you fly. If your flight arrives late and you miss your connection, you could be stuck overnight. On the other hand, United and/or Star Alliance partners (e.g., Lufthansa) have flights that leave after 10pm to a number of European cities (e.g., Frankfurt, Munich, Lisbon, Rome) so there is a possibility that United could get you to Europe in the event of a delay via another route. If you want to check for back-up options, search IAD to VCE for the day of your flight and filter on Star Alliance to see what alternatives might exist if your flight arrives too late.

I'm nearly certain United has just one flight per day in each direction between IAD and VCE. Those flights are seasonal.

I'd see whether there are perhaps later flights to Milan. There are lots of trains between Milan and Venice, many taking about 2-1/2 hours from Milano Centrale. Unfortunately, the train from Malpensa Airport to Milano Centrale takes a good while--I think about an hour.

The next-best option would probably be a later flight to Rome. There's decently frequent express rail service between Rome and Venice. The fastest trains take 4 hours, but Fiumicino Airport is a quicker trip into Roma Termini than what's available from Malpensa to Milan, which makes up some of the difference between train time from Milan and train time from Rome.