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.
Lack of sun exposure: Lack of sun exposure is a major cause of low vitamin d level especially if you live in the northern us. Less likely would be lack of vitamin d in food, but also a possibility if you are on a very restrictive diet.
Vitamin D is primarily made in the skin when it is exposed to sunlight. Few natural foods contain vitamin D. So if you are not drinking liquids (like milk) or eating foods (like cereal) with added vitamin D and you avoid the sun, it's no surprise that your vitamin D level would be low. The low level of B12 could indicate an underlying disorder. But first you need to look at your diet.