The simple view of the bonding in methane You will be familiar with drawing methane using dots and crosses diagrams, but it is worth looking at its structure a bit more closely. There is a serious mis-match between this structure and the modern electronic structure of carbon, 1s 2 2s 2 2p x 1 2p y 1.
The second reason for a low vitamin D is that you have primary hyperparathyroidism due to a small tumor in your neck and the body converts all the vitamin D-25 (that your doctor is measuring) to the active Vitamin D-1-25 (that doctors do not measure). This is the blue blob on the graph.
It is a tumor and it will not go away. Vitamin D levels have nothing to do with it--except to prove that the source of the high blood calcium is your bones, not your diet. The low vitamin-D levels is the body's way of preventing the intestines from absorbing more calcium. The body is protecting itself! Do not avoid parathyroid surgery because your doctor says your vitamin D level is low, and that the low vitamin D is the cause of your problems.