It says that he's 5' 11 3/4'

I think that that's his real height. Previously on IMDB (Like 10+ years ago he was billed as 6 feet tall), but I believe that he's 5'11.

Did you notice how tall he is in the Fast 5 stills or in the movie when he's standing face-to-face with The Rock? The Rock is 6ft 5in, and they made Diesel look a few inches shorter like 6ft 3in or 6ft 2in...

Vin was DEFINTELY standing on one big box there.

Nothing is what is seems

he's 6 ft 11.

proof: https://images.moviepilot.com/images/c_limit,q_auto,w_710/groot-jig-vin-diesel-on-stilts-is-the-best-thing-ever-jpeg-103866/vin-diesel-on-stilts-is-the-best-thing-ever.jpg