Well, I beat it. Great game.

[SPOILER="Ending Summary"]
Ending left me a little confused however, so I did a little reading online and this is the best summary I found...

Everyone's focus was to destroy Orphan. Fang and Vanille's focus was probably to bring down Cocoon (which goes hand-in-hand with the other's focuses).

Because the Maker made the Fal'Cie in such a way that they cannot commit suicide or work against themselves, Orphan had to be overpowered. The only power known to be able to do this is Ragnarok.

Fang's brand is scorched, because in the past, she became Ragnarok and started to destroy Cocoon. The Goddess, an external force that I believe is either a very strong Fal'Cie or on par with the Maker, felt that to extinguish the lives of so many people of Cocoon is wrong, so she froze Fang and Vanille prematurely in crystal. Vanille's brand remains, because as she tells you earlier, Fang was the only one that did anything.

As others have posted above, Orphan used an illusion on Vanille and Fang to show them that the others had turned to Ci'eth. Fang initially becomes Ragnarok and is able to tear down Orphan's shield, but she has a moment of doubt and reverts. Can she really cause the deaths of millions of people? The weight of that sin causes her to falter.

Due to Orphan's nature, it needs Ragnarok to finish the job, even if its shield is penetrated. Thus, it tortures Fang to get her to change back, because it craves death and release and is so close.

The others come in and kill Orphan when its shield is down and apologize to Fang for letting her "go it alone." Together, they shoulder the burden of bringing down Cocoon and killing everyone, because they realize that Orphan won't stop. If they fail, others will be made to replace them and the cycle repeats. So they take it upon themselves to share small portions of the travesty.

At this point, all their focuses are complete. Orphan is destroyed and Cocoon is being brought down, but they've decided that with their remaining power before they turn to crystal, they'll save Cocoon. Just like Serah said. The l'Cie will defy their fate, as Cid Raines had done.

And you know the rest. Vanille and Fang split off from the others. Their time is over. This is the least they can do to repay the kindness of shouldering their despair. They were stranded on Cocoon, lost from each other, and filled with despair. Each of them had an impossible task ahead. And yet, in the midst of all that, Vanille and Fang met the others and were allowed to hope again. They found true friendship and family in the least likely of places. And for that, they show their thanks.

With Cocoon plummeting down towards Pulse at such speed, the sheer magnitude of the impact would destroy Cocoon and devastate Pulse. Seeing this, Vanille and Fang form Ragnarok together. They had the strength to do the impossible all along, but before, they lacked the will to murder so many. To prevent the deaths of millions is easy. With doubt no longer in their minds, they use all their strength to save Cocoon and use their combined power to bring the others out of crystal.[/SPOILER]