I'm not here to hate on these people. What's done is done. They've probably learnt their lesson, and it's none of my business. It could so easily happen again however, and it could so easily be prevented. How can we do this?
I have nothing against babies being brought up as vegans, or breastfed. It's said that babies should be started on solids after six months, but I don't really feel that's the big issue here. The big issue is that the mothers milk was inadequate to support her baby. The mothers milk likely didn't contain enough Vitamin A or B12. This was probably because the mother herself lacked those vitamins. B12 deficiency can be fixed relatively easily with supplementation. If you're not already deficient, oral B12 should be enough to keep you from becoming deficient, but if you're planning to breast feed your newly born baby and you know you're deficient, it's probably best you take B12 injections as they work to fix deficiencies faster. Alternatively you could perhaps pump breast milk, put it in a bottle and mix it with some kind of B12 supplement. Discuss with your doctor. You could also choose not to breast feed and give your child a vegan baby formula. One final option could be to get someone else (healthy)to breastfeed your child.
Vitamin A deficiency is rare in adults. If the mother was deficient in Vitamin A, she may not have been taking in enough, but more likely explanations would be lack of fat in the diet, needed to absorb Vitamin A, or thyroid issues which can apparently interfere with the conversion of plant Vitamin A to Retinol.
Thyroid issues can be caused by lack of iodine, or too much iodine. It's more likely vegans will suffer from lack of iodine unless they consume the seaweed Kelp, in which case they could be getting too much iodine. Most people get their iodine from iodized salt, fish, or milk. There is iodine in fruits, vegetables, legumes and grains, but the amount of it depends on the soil quality.
Some naturalist people insist we don't need to take B12 supplements, because B12 is in plant foods and the air. They suggest B12 deficiency is an absorption issue, and that a fast can fix this. I've seen no evidence to suggest these things. B12 is found in plant foods, but likely in amounts too small. There isn't much data on it. Your body does have bacteria that produces B12, but it's in the large intestine. The small intestine is the absorption point for B12.