My understanding is that Vitamin D is the thing here. There is not much of it at all in breastmilk, even less if the mother was vitamin D deficient herself. It is added to formula.
You get your useable vitamin D from sunlight mainly, food sources are generally pretty poor, we don't fortify food with it in the UK and beside, food has a different form of the vitamin in them, which is not as easily used by your body.
If you are darker skinned or don't get out much or tend to cover up a lot, or live in the UK or other northern latitudes and it is winter (October to April IIRC) you can't get vitamin D from sunlight either as the wavelength of the light is not right. Some places are seeing more cases of rickets now, others more cases of bone aches and pains indicative of deficiency.
I've read a lot on this recently (work). I give my DS vitamin drops and have since he was weaned. I don't think it's an inadequate breastmilk thing at all - formula feeders are already supplementing, just as part of the feed, not with separate vitamin drops. It's just how things are in northern latitudes.
NICE guidance including vitamin supplementation advice has just come out, so there will be more info around on this in the coming months. If you really don't want to give your lo vitamin drops, the best thing to do is to try and ensure your vitamin D levels are high throughout pg and bf - supplement if it's winter, get out in the sun for 15 mins a day around the 11-3pm window if it is summer.