Vin Diesel has just posted this on his Facebook page seemingly confirming that he will do the motion capture for the film which will be released next August. Officially announced at Marvel.com

"Vin Diesel is a super awesome dude. I can't tell you how much I like this guy." “I hope this puts to rest all the silly Thanos-Vin stuff which people just seemed to make up out of nothing.” “He said he wanted to do it and we said we were talking to him. Everything else was people guessing and thinking they knew what was going to happen” 

Gunn also talked about the bust of Groot that Diesel is posing next to.

“This was the bust we used for reference when shooting. It has changed slightly from this since we started animating him.”

Marvel Studios CEO spoke with The Huffington Post today about the current Ant-Man and Guardians of the Galaxy rumors and here is what he said:

We last spoke at Comic Con and his name was in the press then, so I wasn't going to bring his name up again but ... Vin Diesel, what a hoot.

[Laughs] Yes.

He was walking around on stilts.

He's got to keep his 50 million followers, or whatever it is, entertained. It's pretty impressive.
What is he doing?

Well, I think he's filming "Fast 7" right now -- and occasionally filming himself.

He's made so much news with this, I had to double check this morning to confirm that there still hasn't been an official announcement that he's a part of "Guardians of the Galaxy."

Yeah, I think it's close. I think it's close to being official for the Groot character. But, not 100 percent yet.
OK, so he's not nuts.

Well, I didn't say that [laughs]. But, no, and this did not exist when we spoke at Comic Con. That is where we sort of found the potential to do something.

Why is Joseph Gordon-Levitt's name always come up when a new Marvel movie is being cast? This has happened three times now.

Well, sometimes it's because we are legitimately talking, and sometimes it's just because he's a prominent actor out there now. And lots of rumors surround prominent actors.

Paul Rudd's name is out there for Ant-Man. That would be fun if it's true. I have no idea if it's true.

Well, nothing is true yet. I like Paul Rudd, too.