Sep 30, 2014 - 5:38 PM
It's doable if you leave early. The trip from Zermatt to Montreux takes about 2h30, so you could leave Zermatt by 7.30 AM and arrive in Montreux by 10 AM. This will allow you to spend quite some hours in Montreux before heading for Geneva Airport (another 1h15), where you should arrive well in time to catch your flight.
Alternatively, you can travel right on to Geneva and do some sightseeing there instead of in Montreux.
The timetable will be of help when planning further details.