Well Marvel hasn't confirmed that Vin Diesel is going to be in Marvel's upcoming "Guardians of the Galaxy" as Groot or Thanos. But according to this intervew with Chris Pratt even though he says he's afraid of exposing too much about the movie he did say that since he's known that Vin Diesel was going to be cast as Groot that he watched Diesel's movies in a new light.
"I think Vin Diesel is going to be great. The role of Groot is also an amazing role. I want to tell you about it, but I feel like even if I tell you one little thing it's going to start pulling the thread, like expose too much. But I think Vin is going to be amazing. Vin Diesel, his voice is an incredible tool that he has. I've watched a couple of his movies now that I knew he was going to be cast as Groot, and listening to his voice...truly his greatest talent is his voice."
Here's the video from IamRogue
He also says Bradley Cooper is Awesome as Rocket Raccoon(he does look like him doesn't he?)The more I hear about this movie The more I'm looking forward to this movie. How 'bout you?