You're all missing the one thing that stuck me the most about Fang.
She is the definitive military soldier.
Male or female. She will do whatever is necessary to get her objective done. Vanille was her comrade in arms. Soldiers are willing to die for their fellow soldiers at any point. They protect each other, and they never leave a soldier behind. While, yes I did see them as friends, I saw Fang's care for Vanille as that of a solider loving and willing to die for a comrade in arms, and Vanille just being a sweet girl who loves and cares for those who care for her in return. Vanille cared extra for Fang, but she was sweet and kind and loving to the entire group. Fang however felt for most of the game that these new L'cie had a different goal than her and Vanille.
(SPOILER)Even at the Final Boss she was willing to do what she thought her mission was despite everyone else. She was resistant to all of them, while Vanille was not. They were not her allies, they were not her comrades in arms... until of course after everything.
And that would work whether Fang was a guy or a girl.
THE JACKEL