Swiss has 3 nonstops a day from ZRH to VCE. You could take a 9 am direct train from Interlaken Ost to ZRH, arriving at 11:14 am, in time to get a 1:00 pm flight to VCE, arriving at 2:05 pm. then figure at least one hour to get from VCE into Venice. If 1 3/4 hours from the train station to flight departure seems like not enough time, you can get an 8;30 am train with a connection in Bern arriving at VCE at 10:54 am.
Best train route with fewest connections would be an 8:04 train to Luzern (very scenic), then a train to Lugano (also quite scenic), then a train to Milano Centrale, then a train to Venice, arriving at 5:12 pm.
The air route saves about 3 hours, but will be more expensive with a train ticket, expensive airline ticket, then cost of getting into Venice from the airport. Train is one ticket, 3 connections, and delivers you into Venice proper at Santa Lucia station, on the Grand Canal.