Transportation

I'd start with an airline...surely a low-cost route will get you at least close to Venice, then you can train (from Pisa, or Milan, or even Rome). Be sure and check from Paris, too! Also, be sure you know where the airports are; they aren't always the 'main' airport, and may in fact be several miles away from where you think they are. (seems like a Munich>Pisa route went to Cologne first...you might try from Cologne, too!)

Ryanair and Brussels Airlines will each fly you there non-stop. Ryanair of course is much the cheaper of the two, although it flies out of the Brussels South Charleroi airport. There is a shuttle bus to Charleroi airport from the main Brussels railway station.

Fly

Or, if you wanted to go by train, you could take the Thalys to Paris and then the night train. But flying might well be cheaper. See http://seat61.com/Italy.htm#London%20to%20Venice,%20Verona,%20Milan,%20Turin for other options for the Paris-Venice leg.

If you take advantage of the Bahn's Europa-Spezial fares, you could go from Brussels to Venice (or at least to Verona) for as low as €78 (plus an optional €5 for two seat reservations). The total travel time is about 14 hrs. With an advance purchase Sparnight fare you could take the night train from Munich to Venice. A seat on that train starts at €43, an economy single sleeper at €139, or you can stop anywhere in Germany for the night. Augsburg might be less expensive for accommodations than Munich.