That was in 1989 so I don't think that was it.
If the person alluded to in the blind item is GV, my suspicion is the book and documentary were to get ahead of something about to be revealed regarding herself or her family.
I am really only guessing but GV was really out of the public eye for a long time then suddenly she is on a host of talk shows with Anderson Cooper. Why at 92?
Also, why was this article released on Monday, January 21, 2019? The timing of this with the blind item is interesting to me.
GMV was married at 17 (with parental "consent") to a 42-year-old Vanderbilt, and had her only child, GV, a year later. Eighteen months after that, Daddy Vanderbilt died, of "a throat infection which had caused internal hemorrhages". (Sounds like poison to me.) Because GMV was still under 21, there was a huge custody battle for GV -- the "trial of the century" -- until it turned out that GMV was a year older than everybody thought. (This means that she was actually 18 when she was married, so she didn't actually need the parental consent that was given.)
More at the Wikipedia link.