This is rather like asking the question “Do you still beat your wife?” Either yes or no would imply that you have beaten her in the past.
So the choices in your question are that Vin Diesel is gay or he is gay but not admitting it, right? What if there is a third choice, that Vin Diesel is heterosexual? Or a fourth, that he is bisexual? Considering that he has apparently fathered a child with his girlfriend Paloma Jimenez, he certainly has had sex with a woman at least once.
And since the majority of us will never even meet the man, why does it matter? He himself just wants to keep his private life private, and can you blame him? If you want to fantasize about him go ahead. It won’t hurt anyone, and you will never have to deal with the reality that he may be a slob, or a snorer, or whatever.