I will be the first to admit that I don't know enough about nutrition to say much about this topic, however here are my thoughts.
God made me. God made my baby. God made my breastmilk for my baby. In conclusion: The breastmilk that I produce for my baby is sufficient for her needs.
Okay, seriously, I do know a bit more than that. First, it's important for every lactating momma to know that the breastmilk you produce is not only unique to you--it's unique to your baby. That means the composition of the milk you make for you first baby, is not necessarily the same as the composition of the milk you produce for your second, third, and so on. Each baby receives what they need.
Secondly, it is imperative that you have a healthy diet and lifestyle so that you are producing the best milk possible for your baby. Make sure you're still taking pre-natal vitamins, getting good amounts of sunshine, and if necessary, a vitamin D supplement. A good diet, rich in vitamin D is good as well. Here are some great sources:
Salmon (I've got a great recipe, if you're fish-finicky)
Milk (goat's milk too)
Eggs (There are more than 100 ways to cook an egg, surely you like one of them.)
Even though research claims that breastmilk is deficient in vitamin D that does not mean that your breastmilk does not have vitamin D. Possibly just low levels. The best way to get vitamin D is via the sun. I find it ironic that we've been told to avoid sun exposure because it causes cancer, and now the research is telling us that vitamin D deficiency is linked to certain types of cancer. All things in balance, right? Be sure your baby gets 10 minutes of sunshine a day (at least)--no sunscreen, no hat, heck--no clothes. I let my little ones out in the summer bare skin just before the heat of the day (between 10 a.m-1 p.m. for our area). I watch them CAREFULLY, and keep them cool playing in water and dirt.
My son, who has CF, requires much higher levels of all the fatty vitamins (A, D, E, and K). He takes vitamins specially formulated for CF kids. He also spends LOTS of time outside without a shirt and very little sunscreen. We've been doing that since he was a tiny baby, and he hasn't suffered any sunburn. He also eats LOTS of fish--specifically salmon. Our family LOVES salmon, especially O. We got him hooked at a young age on smoked salmon, and now he gobbles up any fish I put in front of him. Also, I highly recommend the fish market at Whole Foods. It's fresh, and they are VERY knowledgeable about their product. They'll answer all your questions and then some. O also likes taking fish oil--eeck! I take mine in pill form, but he ENJOYS the liquid! Yuck, yuck, and double yuck!
At O's yearly CF exam (he goes quarterly, but once a year is the "big one" where they do all the blood work) his vitamin levels are checked. We've always done his exam in April, and his vitamin D levels have been low. I completely attribute that to the fact that we had spent the majority of the winter and early spring indoors. This year we bumped his appointment to May, and with just one month's difference his levels were normal. Now that he's older I think it will be easier to get him outside during the snowy months, but if necessary, he'll get an extra supplement.
I'm definitely NOT against giving kids vitamins, but I also try to let what God has created work on it's own. At the same time I use what He has given me to it's best (eating well, taking care of myself, eating the abundance of things he created to keep us well, enjoying the glorious sunshine, etc.) That can also mean in some cases that a supplement is necessary for good health.
Info on vitamin D and breastmilk: