Apparently he did. Though whether he was in his right mind when he did it is being contested.
It is rather nice to think that Vidal thought the Harvard libraries would be a safe repository for his papers and that the University would use his riches wisely. It is not so nice to think of Harvard profiting from a self-centered fellow who was too mean-spirited and nasty to take care of the relatives and helpers who made his life tolerable during his decline. Some of them are hoping that even if Harvard gets the bulk of the money, it will make some provision to support them for the rest of their lives.
It is a nice set of dilemmas. Would Harvard have a moral obligation to second-guess Vidal's feelings toward his kin? Would it dishonor Vidal to provide for people Vidal quite intentionally disinherited? Would doing so tend to support the position of those who think Vidal was not thinking straight when he brought Harvard into the will in the first place, only in the last year of his life?
It would not be the first time Harvard has received a large bequest and relatives left penniless were distressed about it. Thank you, Gordon McKay! McKay at least provided for some of his dear friends, even if he changed his mind several times about who those were.