A good gut flora ( good bacterias)  is important for better digestion and absorption of nutrients from the food you eat .  Most of the times stress and antibiotics can spoil the gut flora , which we leave unbothered Start yourself on probiotics first. Then eat vit b12 rich food like mutton , fish . Your vitamin absorption will increase. Increase sun exposure for vit d . Vit d is a fat soluble vitamin - therefore make sure you have good fats in your diet as well. Both vitamin absorption will be reduced in the absence of good gut flora.

Early morning Sun light for 15minutes Will take care of D3. B12 you have to take supplement. As vegetarian diet does not contain B12 enough