As you all know, i'm not an oncologist so I don't pretend to have a medical explanation for why vitamin D helps with back pain.  However the b-m.org forum archives are full of articles on the benefits of vitamin D.  And the many myeloma forums out there are full of posts about back pain.

I have undergone bone damage.  I work at strengthing my bones. I suppliment with vitamin D 1,000mg x 2 each day. I exercise.  The study mentioned in the link below is another reason for me to suppliment with vitamin D.  David