Ever since it was announced that Vin Diesel was "in talks" to join 'Guardians of the Galaxy' as Groot, it seemed like the casting was a done deal and that it must have been the only reason that Diesel was meeting with Marvel (which he had teased weeks earlier). According to this recent interview, both assumptions are wrong.

Diesel was asked about 'Guardians' by The Hollywood Reporter, and he was cagey about it, which means the ink isn't dry yet and this is the sort of thing that could fall through. He also talks about doing motion capture for the part. Whether he would do it during or after principle photography, or if he would film on stage with his 'Galaxy' costars Chris Pratt and Zoe Saldana is unknown. But he does make it sound like he expects to do more than just sit in a recording studio.

As for the other point, Diesel says he was meeting with Marvel to talk about perhaps being in his own superhero movie (or sequel), and that 'Guardians' was the only film he could make an appearance in before 2016. That doesn't rule out Diesel getting his own Marvel vehicle, as we've seen Chris Evans and Ben Affleck double dip comic-book characters, so playing two separate character in the Marvel universe isn't out of the question.