Can I Safely Skip Spending a Night in Geneva to Catch a 12:25 Flight?

You should check the train schedule to make sure, but I think probably yes. I did something similar last year: flight at 1:40 PM out of Zurich, spent the previous night in Bern, caught a train at 8:30-ish AM, made it to the airport in plenty of time. Swiss trains are extremely reliable, and Swiss airports are very efficient. If there's a train scheduled to get you to the airport by 10:00-10:30, you should be fine.

Look at the train schedule for that day and you can answer that question yourself. I will be in a similar situation as I will be taking a 1:40pm flight from Zürich to JFK yet will not be staying in Zürich the night before. I'll be in Luzern; there are plenty of trains I can take to ZRH that go DIRECTLY to ZRH.

When you look for a train, pick one that goes directly to the Geneva airport. Make it easy on yourself.

Usually I like to stay in my departure city the night before. However, I agree with the above posters. Lausanne is so close that it wouldn't be an issue, also with the exception of the very special CERN tour, I feel Lausanne is much more desirable than Geneva for a stay.

We spent the night in a lakefront hotel in Morges prior to our noon-ish flight out of Geneva, and that was fine. Many or most of the trains running on that route do continue to the airport after stopping in Geneva, so no change is necessary. The trains can be crowded during morning commute hours, but by 9:30 that was over.

There are 2 direct trains every hour from Vevey and Lausanne to Geneva Airport that take 1 hour and 2 to 11 minutes (from Vevey, less from Lausanne). There are also other trains with a connection. The Swiss count on the reliability of their train system, I think you should too.

The first reply here suggests 2 hours before flight time at the airport.

I expect that your airline to Canada expects you there 3 hours ahead.

What does your airline say?

Unless there is scheduled engineering work on the line, which is EXTREMELY unlikely on a weekday, there is absolutely no problem. You'd have to leave Vevey at 8-ish. Expect crowded trains, however.

I am not a Canadian citizen, but the U.S. requires that airlines sending planes to the U.S. to provide a confirmed passenger list well before the arrival of the plane. That is one of the reasons U.S. airlines have a 3-hour check in rule for transatlantic flights. Of course there are other reasons, like saving money on gate rental and staff. Economy Class checkins at major European airports often have very long lines, especially in peak tourism months and around holidays.