Vin Diesel just posted an image of Groot from Guardians Of The Galaxy to his Facebook page. While this isn't exactly a confirmation that Vin Diesel is playing Groot in Guardians Of The Galaxy, it's certainly a strong hint.
Vin Diesel previously revealed on his Facebook page that he had a meeting with Marvel Studios, which set off a ton of speculation over which role he might be up for. Most of the early hints from Vin Diesel pointed toward him playing the Vision in Avengers 2.
At San Diego Comic Con, Vin Diesel said that an announcement concerning him and Marvel Studios was coming soon, which led to some speculation he could be playing Thanos in Guardians Of The Galaxy, since filming is currently under way for that movie. There had also been some speculation that Vin Diesel might be up for the Rocket Raccoon role, but there hadn't been much speculation about the possibility of him playing Groot.