Judging from the comments so far, not many post-Broyard celebrities have been revealed to have actively passed from black to white. 

I can think of a number of part-black actresses who switch back and forth between white and black roles:

Rashida Jones — "Parks and Recreation" — Her father is Quincey Jones

Maya Rudolph — "Saturday Night Live" — Her mother was the late singer Minnie Riperton and her father is an entertainment industry insider, friend of Spielberg, etc.

Rebecca Hall — Ben Affleck's girlfriend in "The Town." Her mother is Maria Ewing, a part-black opera singer.

Jennifer Beals — "Flashdance"

The first three are daughters of people in arts & entertainment. Quincey Jones, producer of "Thriller," may well have an entire trophy room for all the awards he's won. Rebecca Hall's father, Sir Peter Hall, was knighted for being one of the top British stage directors of his generation. So, their daughters have incentives to play up their ancestry.

There are more white roles for actresses, and nonwhites tend to jealously guard the nonwhite roles. So, the rule of thumb appears to be that you have to identify as part black to get the black roles, but that won't stop you from getting the white roles, especially if you are show biz royalty.

In the case of Vin Diesel ("Fast & Furious"), his Wikipedia article currently says he says he's part black, but I vaguely recall him not saying that a decade ago. His family background is complex (he's never met his biological father, his step-father who raised him is black), so if my recollection is right, I would guess he'd fall in the personal reasons category. (By the way, I'm always struck by how similar his facial expressions are to Jerry Seinfeld's.)

In contrast, sexual orientation works somewhat differently. Gay actors who want leading man roles are discouraged from coming out. One late-blooming action star, who used to make most of his money gentrifying homes, has been plagued for several years by an ambiguous article that appeared in the L.A. Times real estate section just before he hit it big describing the spectacular renovation he and his "partner," another male actor, did of a once-moldering mansion that they were now putting up for sale for millions. I'm sure he wishes now that the newspaper had put "business" before "partner."

Both straight and gay audiences seem to like having straight male actors play gay roles — e.g., Tom Hanks winning an Oscar in "Philadelphia." Straights are reassured, and gays like that it Undermines Stereotypes, plus they get kind of a thrill out of the idea that a regular guy could be talked into it.