Well I am not a doctor or scientist. Your calcium levels at the moment are “with in” range. I found this statement interesting “there is no sure means of predicting the serum calcium level, for either hypocalcemia or hypercalcemia, at which symptoms will occur.” It comes from emedicine.medscape.com/article/2087447-overview.
So I think you should proceed this way. Your Vitamin D levels are “low”. We recommend maintaining a level of 50ng/ml in order to be and stay healthy. Studies already show cancer rates increase at levels under 30ng/ml. It takes most average weight people (150lbs) taking 5000iu a day to reach this level.
I would recommend reading http://www.vitamindcouncil.org/about-vitamin-d/vitamin-d-and-other-vitamins-and-minerals/. (Taking Vitamin D if you are low in magnesium may cause symptoms.)
Here is the important part! If you start taking Vitamin D and start having any of the following symptoms then you might have hypercalcemia: feeling sick or being sick, poor appetite or loss of appetite, feeling very thirsty, passing urine often, constipation or diarrhea, abdominal pain, muscle weakness or pain, feeling confused, feeling tired and bone pain. Have this and you would need to see a doctor. (Muscle cramps/twitching is probably a magnesium deficiency.)
So I would definitely give it a try!