Whatever was discussed between actor Vin Diesel (Fast & Furious 6) and Marvel Studios President Kevin Feige is so far down the line Feige says it might not ever exist. So that most likely means Vin is not voicing Rocket Raccoon or Groot and probably isn't Ultron.

Vin Diesel has teased a looming Marvel Studios announcement for the past few days but Marvel Studios President Kevin Feige has just thrown a bit of a wet blanket on that wildfire. Said Kevin Feige,Feige also says that no announcements will be made about Vin's role at Disney's upcoming D23 Expo next month. When specifically asked, Feige replied,So if we apply a bit of logic to what Feige says above (who opened the interview with a statement that he would never outright lie à la Talia Al Ghul or Khan) whatever was discussed and agreed upon with Diesel is so far down the line Marvel is unsure if they'll ever even get to it. So that means no, Vin's not voicing Groot or Rocket Raccoon, and no, he's not playing Ultron in "Avengers: Age of Ultron."