We will be taking a train from Venice - Domodossola, then Domo to Lauterbrunnen. How much time do I need to leave between arrving in Domo and departing from there? Don't want to cut it too close, but also don't want to spend much time in Domo either. Been there before :).
15minutes should do it.
Domodossola is not a necessary change point on all Venice-Lauterbrunnen connections, but is recommended for some of them. In that case, DB schedules suggest it can be done in 4 - 5 minutes. I would not doubt that, as long as you are prepared to jump off the train as soon as it stops. If you miss the connection, the next departure is usually in one hour.