Siena - Florence - Venice via bus and train

2 hour buffer you never no. There is a rapido bus?

Why not something like this on the train
13:18 Siena
14:50 Firenze Santa Maria Novella (Florence)
RE 11764
40 min transfer time
15:30 Firenze Santa Maria Novella (Florence)
17:35 Venezia Santa Lucia (Venice, Main Station)
FR 9430

Yes, it's no stop from Siena, 1 hour 15 minutes.
http://www.tiemmespa.it/var/ezdemo_site/storage/original/application/13f322a913848e13eab0b5e3c1eb68e1.pdf

The bus station is on Via Santa Caterina da Siena, next to the Bar Deanna.

Assuming you have no luggage, you need about 5 minutes to walk from the bus station to the train platform inside SMN station. Give yourself extra 10 min if you are retrieving luggage from the bus belly and are walking with substantial luggage.

You should try to be on the correct platform 10 min prior to departure. Few trains to Venice originate from Florence SMN, most come from Rome and pull into Florence 10 min prior to departure time.

So 15 min between bus arrival and train departure is the absolute minimum I would give myself. Actually 20 min because s... happens.
Be aware that if you go in the morning, the commute traffic may affect the bus’ on time arrival. I presume they factored that into the schedule, but I don’t know, and road commute traffic is an unknown variable everywhere.

Thanks Roberto, very helpful.

Jazz - I don't understand what you refer by RE 11764. From the schedule I referred to all the lines begin with '131'. Is there some other?

I was just looking at samples of the train schedule from Siena to Venice to get an idea if it is better to take the train.
RE 11764 is the Regionale train Siena to Florence
That is scheduled on January 30

Jazz was giving you train choices, by RE he meant Regionale train.
Besides by bus, you can also travel from Siena to Florence by train.
The disadvantages are that the Siena train station is not in the city center (which is on top of the hill) but down at the bottom of it. Also the rail line is not a straight shot like the freeway, therefore the train journey is 90 minutes vs, 75 min on the bus.
The advantage of the train is that road traffic doesn’t affect trains (although delays are possible with trains too) and also the train drops you inside the SMN station, instead of 5 min walk from it, therefore all you have to do is change platforms.
There are 16 tracks at SMN.
The tracks at the center are used for high speed trains.
The western tracks (1 to 5) are used for Regionali to the direction to Empoli (Pisa and Siena) and Prato (north and west).
The Eastern tracks (12-16) are used for Regionali for the Arezzo direction (south)
So you would arrive to a low number track and will need to move to a couple platforms to the center.

Grazie!