Breastfeeding is not incompatible with veganism.
The main principle of veganism is not wanting to use animals. Not for food, but also not for clothing, labour, entertainment etc. It's about ethics, not about nutrients.
Of course breast milk is an animal product, but it's not taken from an animal. It is freely given by the mother. That doesn't violate vegan principles at all.
By the way, as far as I know there are no 100% vegan formulas. Even soy based formulas contain some animal derivatives.
If you're serious about this and not just trying to start a silly debate, I recommend the book New Vegetarian Baby. It has a lot of solid information about how to plan a balanced vegetarian or vegan diet for very young children.