i was vegan the first few months of BF (for years before that too) but decided to eat organic dairy and eventually i was a total omni again!
it was tough but i am glad i made the change. now at 27mo of nursing, i am trying to go back to a mostly vegan diet w/ some fish.
i found it easy during pregnancy but while nursing i felt very tired, sick and was underweight. i always ate healthy vegan food (not a lot of processed crap) but it just didnt cut it. sorry i dont mean to be negative at all, just sharing
FWIW I totally think it can work for some people.