In real life Vin Diesel is defiantly bald. I saw him on a talk show with a NW6 shaven down where you could see slight horseshoe stubble. In just about every movie he stars in he has some sort of hairline though; likely drawn on with make up (similar to an smp look). I've also seen him with a buzz cut look in a few movies, not sure how he pulled that off, likely some product that took a few hours to apply and not available to the general public. I got sick of hearing everyone say "oh you're going bald, just shave it off like vin diesel" so one day I looked up a bunch of his movies on youtube and in every one he had a hairline. The only movie I've seen him without any hairline was the new fast and furious (I think the pic that fred posted was from that movie), some of the older fast and furious movies he had a hairline though.
The rock is probably a NW3a. He actually looks great with a shaven head though. But the dude is so damn big, that fact overpowers any hairloss he has. The rock is so big he makes Vin Diesel look tiny standing next to him. When your that big its the first thing anyone notices so hairloss isn't as much of a factor.