Paris to Venice by train

What time is your flight out of Venice? Most flights to the US are actually to a change point somewhere in Europe for the transatlantic flight.

Most of those first flights leave Venice very early in the morning causing difficulties on that last morning.

I'd go 4 nights in Paris and 4 in Venice.

If you add two nights somewhere between the two are you going to cut nights in Venice or Paris?

How about just Lauterbrunnen or Mürren (my preference) in Switzerland? That way you only lose two days to traveling.

With 8 nights total you could do 3 nights in Paris, 3 nights Lauterbrunnen and 2 nights in Venice.

Lakeside Lausanne or the historical capital of Bern in Switzerland - both with direct links from Paris by TGV.
Across the Alps into Italy and Milan for the Duomo and Last Supper,
Finally to Venice, perhaps with a few hours in Verona for lunch on the way.

Paris (3/4) > Lausanne (1) > Milan (1) > Venice (2/3)

Nigel - yes, back through Paris and yes we would cut those cities short by 2-3 days. I like the idea of Lauterbrunnen - thanks

With 2 nights in Lauterbrunnen, that gives you one full day. What if it is raining that day? Luzern would take less time to get to from Paris and less time to move on to Venice and give you more rainy day options.