Oerba Dia VanilleSerah has a great impact on Vanille, and as the narrator to Final Fantasy XIII, Vanille’s respect and admiration for Serah is often repeated. Vanille narrates the game to Serah. She’s the addressee. Because Vanille believes, the player does as well.
Both scared and alone, Serah is the one that reaches out to Vanille in her initial isolation in the strange land of Cocoon. Though Vanille is deeply troubled, Serah doesn’t dig about where she’s not wanted - she simply helps, teaches, and is kind. It’s this kindness that greatly touches Vanille, because were it not for her indecision in fulfilling her own Focus five hundred years hence, Serah never would have become a l’Cie - Serah and Dajh become the source/aim of a lot of Vanille’s guilt, incidentally. Even unware of Vanille’s involvement, Serah never casts about to blame anyone for Anima’s branding. And though she might not understand her apology, Serah forgives Vanille anyway.
It’s that big, tender heart that gets to Vanille. Serah’s will, and the fact she beats her Focus, is what gradually inspires Vanille to do the same. She’s the third and last in the line of those with a vested interest in recovering Serah to life because she knew her, and she believes in what Serah wanted. It is certainly better to seek a new path than the alternative, with a Focus that Vanille has put off once before and has been instructed to complete once again.
Serah is the source of Vanille’s policy on face it later, a thought that she took and moulded into an ideal which Vanille later passed onto others. But for Vanille, it was a corruption, because though she put off facing the truth until later, she never got around to the later part.
Though Vanille and Serah only have a momentary impact on each other’s lives directly, they stay linked - honestly, I wish there was more of them together. Even so, I’d hasten to liken them as ‘soul sisters’ because a single meeting bonds them for life. In her narrations, Vanille talks to Serah often, likely for that same connection. Though they are not the same, though their convictions are not entirely alike, they would be the best of friends if given the chance.
And in some ways, they are. When Vanille is set on the soulsong and trapped within the Order of Salvation in Lightning Returns, her only counsel and friend is Lumina. That Lumina picks Vanille to befriend, that they become ”thick as thieves”, makes sense for two reasons - Lumina would understand Vanille’s qualms with her own habitual lying and unintended deceit, because that’s what she’s made of, and second, she has ownership of Serah’s soul. If there was any person Serah could reach out to indirectly - because, at this time, Vanille can hear the laments of the dead - it would be Vanille. Besides, she’s encouraged Vanille in a difficult position before. No one deserves to be alone.