In Chocolat how does Vianne know about Josephine through chocolate? What does she know about her?
When Vianne first sees Josephine Muscat, she notices the same things the rest of the town sees - head down, shaky hands, darting eyes, and the look of a "victim." She discovers her "favorite" chocolate one day when Josephine comes in and discreetly steals it. Vianne knows right then that there is more to Josephine than the victim that everyone else merely pities.
It takes her much longer to get through to Josephine than many of the other characters, but when she finally does, she sees that Josephine has a strength inside that is not dead - but has merely been repressed since she's been in an emotionally and phyisically abusive marriage. Josephine is also stricken with immense guilt because while she knows her life could certainly be better, somewhere deep down she once loved her husband, and continues to hold on to the hope that things will get better.
Vianne, without pressing her too hard lest she scare her away, eventually succeeds in giving Josephine the encouragement she needs. Listen to Vianne's description of her when Josephine finally decides to leave her husband (I think it sums up the answer to your question pretty well):
Her eyes were knives. For the first time since we met, I saw Josephine as she was before ten years of Paul-Marie Muscat had made her wan and ungainly. Half mad with fear, but underneathe the madness, a sanity that chills the heart.