Hello,
Vitamin D is inactive until the kidney converts it to an active form with 1,a-OHas. PTH gets released when there is low serum Ca (someone has osteoporosis. It is needed to activate 1,a-OHase in the kidney. Activated Vitamin D allows the kidney to reabsorb more Ca (bring up serum Ca levels) and secrete more P.
PTH also acts on the gut to increase Ca and P reabsorption from the diet and acts on the bones for osteoclasts to release P and Ca (both these elements are in the bone and you cannot release just one, the bone gets universally broken down). Overall all these actions increase serum Ca levels and you would get feedback on the parathyroid to shut off PTH.