Not to but into this discussion but to but into this discussion, *ahem* you can't decide who the protagonist is, its given through the work. Also the protagonist isn't the most important character but the one that sets everything into play and moves the story along, that was Lightning. She got Sazh involved, she had the goal of killing the falcie or lcie or whatever the gods were called (been a short while). Vanille was kidnapped and saved by Lightning, Fang gave in and became Ragnarok etc. Lightning moved along the entire story cause she was the given protagonist, no other character could of made the events occur in ffxiii as Cute Eclair did (I prefer Eclair cause it always makes me think of her being filled with cream.
Yeah, except everything you mentioned about her/the story is wrong.
The one who prompted the Purge was the Sanctum after the Vanille and Fang's awakening from crystal stasis made the Sanctum find the entrance to the Pulse fal'Cie' Vestige near Bodhum (in the meantime Serah became a l'Cie and got captured be the Pulse fal'cie). The Purge made everyone who was in Bodhum involved, Sazh and Lightning were both there together, don't see how Lightning got Sazh involved.
Her focus was not to kill anyone, the whole party's focus was to destroy Cocoon, everyone being a Pulse fal'Cie.
When was Vanille kidnapped and saved by Lightning again? These two actually have almost no interaction in the game.
Lightning moved nothing even later on, everything was pretty much planned and decided by the fal'Cie (Barthandelus) and the whole party seemed it was going along with it, until the end... when they decided they can choose their fate and not to be any God's slave (which is like the main theme of the Fabula Nova Crystallis games).
Fang decided to became Ragnarock by herself (at first) to not get Vanille involved.
Also, if I remember correctly in the XIII Ultimania they mentioned that they were considering Vanille as the "official" protagonist, but later they decided Lightning was a better rapresentat for the game, so I can only assume that they didn't do big changes on the story after their decision, it would explain everything.
I don't hate Lighting or anything (I like her quite a bit in LR), but I'm just saying it's really hard for me to view her as the protagonist in XIII, even if SE says it. If she is, then the whole party is as well.
I'm also ok with her being the representant in Dissidia lol.