Travel method from Siena to Venice

Train. These sites provide schedules

http://www.ferroviedellostato.it/homepage_en.html

http://www.bahn.de/international/view/en/index.shtml

That's far enough that you would definitely want a train rather than a bus. Take a taxi to the Siena train station and go from there.

Another option that may schedule better is Bus from Siena to Florence, and Train from there. The train from Siena gets to Florence, but not by the most direct or fastest route. Another option, probably no faster, is to bus to Chuisi, then Train to Venice.

I did this route and I suggest bus from Siena to Florence and train from Florence to Vienna.

The bus station is housed just beneath the train station so it is very convienient. I think expense and time wise - bus/train combo is the way to go.

If you have any time to kill in Florence before your train leaves - you can check your bags there and go explore.