Rome to Venice via Val d'Orcia , Tuscany in 1 day

Rome to Venice with a Tuscany visit all in a day? That is unlikely.
You could break it down and spend one night in Tuscany.
The best way to do it is to rent a car (at Roma Tiburtina or Orvieto) then drive through Tuscany. Spend the night somewhere in Tuscany, then the next morning drive around a bit more then head to Venice. You could drive to all the way to Venice, or, if you prefer, return the car in Florence, then take a high speed train to Venice.
If you travel alone the cost of train is similar or slightly cheaper (assuming you don't buy discounted train tickets months in advance in which case the train is definitely cheaper). If it's two of you or more the car is cheaper. In terms of time of travel, that is similar. The train is faster than the car, but once you factor in the rental car return in Florence and the transfer to the Florence central station the time advantage of the train is gone.

Personally I wouldn't attempt this.
BUT
If you got a very early train from Rome to Florence and then picked up a hire car for the day, you would have a large chuck of the day to drive & explore Tuscany, before returning the car and catching a late train to Venice. Very long day though.

Thank you for your response Roberto and djp_syd.

Roberto - I liked your idea of staying overnight in Tuscany. I can see Val d'Orcia is around 2 hours from Rome.
I can leave from Rome around 5-6pm and sleep somewhere in Val d'Orcia. Spend early next day driving around and later drop off the car in Venice.Is there an advantage dropping off the car in Florence and taking a train from there on ?

djp_syd - I can also take the late train to Florence, stay overnight in Florence and and rent a car, the next day morning to drive to Val d'Orcia.

wavering mind...is the post card Val d'Orcia worth it ? Are other Tuscany attractions worth more?
I am not a big art lover.
Should I instead take a day trip to siena after getting to Florence previous night. Many tour companies do that.

What do you recommend?

First ask yourself these 2 questions ...
1 How much time can I allocate to this idea?
2 How do I prefer to travel - drive myself, or on an organised tour?
You really need to proceed from how you think you will get the most enjoyment from the experience.
If you have a night tp spend, perhaps then ...
Rome to Orvieto by early train.
Pick up rental car and drive thru Val d'Orcia to Siena.
Overnight in Siena (and return rental)
Afternoon train to Venice via Florence

The Val d'Orcia is gorgeous. A day there will be a nice break from the city and the countryside is unique. Since you are doing Rome and Venice and are not an art lover, skip Florence.

If you like the idea of taking the train to Chiusi--having a local guide pick you up in her car and take you around the area--then bringing you back to the train station in Chiusi--try contacting Sibylla Holtz by email. I've heard she is good and it could be a nice/fast/knowledgable way to see the Val d'Orcia with just a little time and not much hassle.

I see she is quoted in this article: http://www.nytimes.com/2014/05/11/travel/echoes-of-history-at-a-tuscan-estate.html

Her email: [email protected]