These results provide a direct evidence of disrupted vitamin D metabolic homeostasis in the preeclamptic placenta...
So that bit in the abstract where they talk about genes and upregulating and downregulating? What that means in practical terms is this: so far as the studies show now, we go into pregnancy with a normal distribution of vitamin D (we look just like everyone else) and then as the placenta develops, it begins to sequester vitamin D, probably because doing so turns down some maternal immune system responses, which helps the placenta to hide from us.
The stuff about binding protein sites just means that the placenta is producing extra of a chemical that locks up vitamin D so that it's not bioavailable any more. This is probably why supplementing D doesn't seem to help - the placenta would just respond to a change in the concentration of vitamin D in the bloodstream by making more binding protein.