Gore Vidal died in 2012, but ten years earlier, he had selected Harvard University's Houghton Library as the place where his papers would be archived after his death. (See Gore Vidal’s Papers to Be Housed at Harvard’s Houghton Library, The Gore Vidal Pages, August 2012.)
However, this did not imply that the author's papers would soon become available as a searchable archive that may be accessed by scholars interested in publishing unfinished manuscripts (such as the possibly unfinished manuscript on the Mexican-American War). Eugenia Williams's article Paper Trail: An Abundant Gore Vidal Collection Gets Archived (Boston Magazine, 4 March 2014) explains why scholars and other interested readers need to be patient (emphasis added):
As Vidal’s gift wends its way through the legal system (his half-sister has challenged his will), some of his papers sit in boxes waiting to be cataloged in the basement office of Houghton curator Leslie Morris. (...)
When an archive arrives at the library, Morris and her colleagues roughly sort it into boxes. Hence the present disordered state of Gore Vidal’s papers, which came to Harvard in nearly 400 cartons.
If the manuscript is in one of those almost 400 cartons, access to it will only be possible after the legal issue has been resolved and those cartons have been catalogued. This may explain why Vidal's manuscript about the Mexican-American War has not been published yet (assuming that it is in a publishable or near-publishable state). (The Houghton Library didn't celebrate the bequest until November 2018.)