oerba dia vanille
It's easy to learn a lot about Vanille just by watching the way she acts. She's all smiles, her infectious positivity never faltering. She's kind to others and compassionate. She's headstrong and can be stubborn, but she's also understanding.
You'll notice that these are largely positive characteristics, and if you're simply observing what she shows to others, that's all there is to her. In truth, though, there's a lot more to Vanille that she doesn't express, preferring to keep her feelings to herself in order to protect others.
This hiding comes into play most in her interactions with Sazh and Fang. With Sazh, Vanille hides her origins and her involvement in his son becoming a l'Cie. Up until that point, Sazh and Vanille had been the more light-hearted characters, leaving the seriousness and angst to Lightning and Hope, but in Nautilus they both demonstrate that there's a lot more to them. Vanille had understood what she and Fang had done once Sazh told her about Dajh, and was left to guilitly continue on with him, unable to confess. It almost ends very badly when Sazh finds out, but luckily they're able to make amends.
The secrets Vanille keeps from Fang are also very influential to her development and in fact are the basis for much of the game's conflicts. Vanille doesn't tell Fang that she remembers their Focus and the events that led to their crystallization, and as a result Fang goes to great lengths to determine it. Dajh becomes a Sanctum l'Cie due to their attack on Euride Gorge, and Serah wanders into the Bodhum Vestige after their awakening opens it up. These two events directly or indirectly set all of the other characters' stories in motion. Vanille realizes that instead of protecting people, her lie caused suffering.
Naturally, the truth isn't kept from Fang forever, and she confronts Vanille, having caught to Vanille's deception. They end up having to fight (and defeat) Vanille's Eidolon, called forth due to Vanille's turbulent emotions. Fang forgives her, and Vanille is able to realize that her biggest mistake was not trusting Fang about in the first place.
In the end, Vanille has faced and owned up to the mistakes she's made, and when the moment comes, realizes how to complete her Focus. She and Fang become Ragnarok together, and their combined powers save Cocoon from falling to Pulse after Orphan's destruction. Vanille has come a long way from the girl who once turned away from completing her Focus. She chooses to save Cocoon instead of destroying it, and in so doing returns to the way she was at the start: a crystal. This time, though, it's because she successfully did what she set out to do so long ago.