Hello
Vitamin D is important for many functions of our body. I would encourage you to focus on natural sources of VitaminD like curd, milk, egg and most important of all - SUN. Try to go out for a walk when possible.
For taking the multi vitamin you may need a prescription and should ideally consult a physician in your city.
Next Steps
See a physician who would suggest option of oral tablet everyday or once a week type of medicine. Other option would be injectable.