Do you have access to a holistic practitioner or orthomolecular practitioner? I used to work for one, and we had vitamin D that was veggie sourced.
the other thing you may want to inquire about is a Vitamin D injection. I know its kind of hard to find docs that will do this but it may be a good idea in your case.
Hope things get better for you-my sister is going through this and she is just miserable/exhausted.