Not sure I can help much, but since she will take water, push fluids, especially when she eats prunes (because they are dehydrated, she needs to get the water that was taken out of them, or it could cause problems).
I doubt it's a vitamin deficiency. But then I'm not a doctor. I do understand a little of how fiber works, however. Fiber is bulk. You can see it in celery, but most of it is too small to see. Anyhow, this bulk is kind of like a broom for the intestines. It also acts as a sponge, so that things don't get too dry (if you drink enough water). So fiber isn't really absorbed.
My big recommendation would be to try cutting out dairy completely. Not your breastmilk, but other forms of dairy. That would include dairy in your diet, since she's getting it through the milk. You didn't say if you limit the above-listed things in your diet or not. It might be a good idea to, at least for the dairy. Sometimes the casein in cow's milk can cause constipation for children. Most people don't know this, but casein is very species specific, and cow's casein is very difficult to digest, even for adults. Considering that she is breastfed, she is getting plenty of nutrients from you, and you can find all kinds of replacements for milk, like soy milk, rice milk, hemp milk... Seriously, try it for a few weeks and see how it goes.
I hope you figure it out!