B.T.
Don't let people make you feel bad about breastfeeding. I don't know why they discourage the healthiest form of nutrition for babies. I have come across that negative reaction myself. I breastfed my son when he was a baby and am still nursing my daughter. I had the opposite problem with the vitamins though. My mom and MIL thought it was weird when our pediatrician told us to get PolyViSol for our son at about 4 months. At 6 months, we were given a same-type of vitamin, only it also has flouride. I was told Vitamin D does not pass through the breast milk. That is why they now suggest the vitamin. I did it with my son and now with my daughter only because I trust my pediatrician. I would take much of what your MIL says with a grain of salt. Times have changed, and doctors learn new ways to do things all the time. I would definitely talk to your daughter's pediatrician or nurse.
By the way, my mom (who nursed my youngest sister until she was almost 3 and she started getting teeth about 4 months) never supplemented any of us with a vitamin while nursing and we are all healthy.