Paris-Zurich-Milan-Venezia Santa Lucia, departing Paris at 06:24 and arriving at Venezia Santa Lucia at 17:40. Book Paris-Zurich at www.tgv-europe.com. To keep the site in English and to avoid being bumped to the Rail Europe site which doesn't offer discount fares, choose Great Britain as your country of residence. If you book today, you should still be able to get a Piccolo fare of 62 EUR. Pick up your tickets in Paris. Book Zurich-Venezia Santa Lucia at www.sbb.ch/en. Choose the 11:09 departure from Zurich. That gives you only 9 minutes to make the connection after you arrive in Zurich, but it's enough. Be standing at the door when you arrive so you will be the first people to get off the train. You should be able to get a discount fare of 78 CHF.
Paris-Basel-Milan-Venezia Santa Lucia, departing Paris at 08:24 and arriving at Venezia Santa Lucia at 20:02. Book Paris-Basel today at www.tgv-europe.com to get a Piccolo fare of 53 EUR. Book Basel-Venezia Santa Lucia at www.sbb.ch/en, departing Basel at 12:28 and arriving at Venezia Santa Lucia at 20:02. (Choose the route that does not require you to change trains at Venezia Mestre, the mainland station.) You should be able to get a discount fare of 78 CHF.
When you book your tickets on the Swiss Rail site, choose to have them mailed to your home. You definitely won't have time to retrieve them in Zurich and in Basel you might run into trouble if the lines are long. You'll have to pay a fee, but you'll have the tickets in hand.