This theory has as much reasoning behind it as suggesting that Vanille is actually Snow's mother with a sex change. While some theories of yours might have some vague reasoning behind them, this one is right out there, I have to say.

FE, I suggest you read the short story translations which give you a much better insight to how the characters think and also a slight insight as to how they grew up together from a young age. They were children together so Fang can not be Vanille's mother.