With the measure of hindsight, and seeing it put like that, I'd say they used Vivi's plot as a way to basically flesh out Zidane's plot while also keeping the main twist centered around Zidane without giving it away.

The thing that drives me batty about Zidane though, is that he's always put his kindness and faith in others, and the whole, "You're Not Alone" segment is just so drastically out of character for him to me. But not only that, the more you analyze his character, you realize it's more of a contrived feel good character rather than a person (as is that scene). He's supposed to want to get on like a bro from Tantalus, a nobody thief from a band of brothers, but the ending result of his character arc is that he's actually some sorta Kuja 2.0 that's perfected, super dee duper powerful, and that's the actual reason for his extreme competence. He was genetically engineered with it. I don't remember if his memory loss about his origins was explained either, but he's more or less a Superman/Goku style dude.

Granted, main character with an epic tie in to the setting's past has been a staple in FF since FFIV, I just really don't like how it was handled with Zidane.

FFIV had Cecil and Golbez as Brothers, pretty much a straight lifted Darth Vader type thing (Maria from FFII also experienced this but she was technically not THE main character).
FFV had Buttz related to one of the OG Crystal Warriors that initially sealed Exdeath (and this becomes true of the whole party actually).
FFVI had Terra be half Esper and Celes be Esper infused through Magitek (covering both starting characters for WoB and WoR respectively).
FFVII has Cloud's whole amnesia/Soldier/Zack's persona infusion thing.
FFVIII had Squall be related to Ellone and Laguna and such and the Amnesia orphanage run by Edea too.
FFIX has Zidane be Kuja's "brother" as well as a "soul-less" Genome as well as amnesia about it.

So it's par for the course for the series, but I find its delivery in IX to be a tad hamfisted, with the important story beats around it already expressed better by other characters, while it also makes Zidane's whole, "My friends are my strength." thing with Amarant ring hollow.