The Rock's father is an Afro-Canadian former pro wrestler (Rocky Johnson) and his mother is a Samoan chief and pro wrestler's daughter (Ate Maivia). I think that the King Kamehameha biopic is still in the works. It's supposed to be his pet project. As far back as I can remember in the Rock's choice of roles, he is either openly black or implied to be black, such as his Hobbs character in the F/F series. Remember his early comedy role as the black gay aspiring country singer in Get Shorty or Be Cool (I forget which)?
As far as Vin Diesel is concerned, I agree that he downplays race as an ingredient to his character. In fact, I don't recall Vin Diesel ever playing an obviously black character. In fact, his first movie role was a quasi-documentary called Multi-Facial, where he documented his attempts to get roles as a biracial/ambiguous actor and he has since gone on to exploit that, even playing Italian characters (he's half Italian) in several films, not just Saving Private Ryan.
I guess to me the difference is The Rock chooses a variety of roles where he is either black, part black, some other nonwhite ethnicity or unstated, and Vin Diesel is usually Italian or ambiguous, but never black.