My first thought was "have her level retested to make sure the baby truly has low vitamin D and it wasn't a lab error." But then you listed various symptoms, so it seems likely that she truly does have a problem. I'm not sure there would be any benefit to redoing the test, so why put her through that?
6mo babies don't just spontaneously have problems with vitamin D; "normal healthy" babies get plenty of vitamin D prenatally, plus they get small amounts via breastmilk. They don't get all they NEED from milk alone, but a good prenatal store of D lasts them until they're ready for solids. There's some reason her vitamin D stores didn't last her- the most likely is that she didn't put enough stores down prenatally because you're deficient (it's also possible that you have plenty of D and there's some kind of metabolic problem with the baby, but that's far less likely.) So, take lots of vitamin D yourself, for yourself, even though that's unlikely to be enough to solve the baby's problem.
It's also a good idea for both of you to spend some time in the sunlight, as that's how our bodies are designed to get vitamin D (and the reason the levels of D in breastmilk are so low to begin with.)
The baby also needs to be supplemented directly with vitamin D. I wouldn't use the kind made by formula companies and marketed to babies; those have synthetic additives (flavors, sweeteners, and possibly colors too) that can cause their own problems. I'd go with an oil-based supplement: either cod liver oil, those liquid D drops somebody mentioned above, or even vitamin D capsules intended for adults that you prick open and squeeze the oil into her mouth or onto your nipple before nursing (or mix into solids or put into a bottle, if she consumes anything other than milk directly from your breast right now).
I would have her levels retested regularly; besides making sure she gets ENOUGH vitamin D, it's important to make sure she doesn't get an overdose when you're supplementing. If that happened, you'd simply stop the supplement for a while and test her levels again.
ETA: if vitamin D shots are a way to get her levels up quickly, that's something to seriously consider. It's not something I've heard of before, so I didn't mention it until I read your post about your doctors recomending it. But I hope that would be a short-term solution; something to quickly get her up to baseline, then you'd use oral supplements and sunshine to maintain the level.
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