Having breastfed two kids, my experience was that it was important to keep calories up as well as good fats (avocado, coconut milk, nuts, etc.). You may be hungrier than normal and needing to eat more--definitely honor that!
There is also a section in the Whole30 book specifically on breastfeeding--it's not specifically tailored to vegetarians, but a lot of it is generally applicable regardless of your specific dietary context.