Hi there, Rick @Rphil2! And, Happy Birthday 
and @katers87
I’ve been on prescription and otc Vitamin D since my diagnosis. It has helped -me- a lot with insulin sensitivity. The traditional course is rx 50,000 iu weekly for a few months, in conjunction with daily otc 2,000 iu. Then, after the few months, the rx is discontinued and otc remains. That was never enough for me and my numbers and health reflected it. I was continually in a loop of on again off again prescription Vitamin D treatment, until a few years ago when I requested to remain on the rx dose, along with the otc. It still isn’t enough. I’m now on 50,000 iu two times a week, plus the otc of 2,000 iu daily. My last lab result was 30, just within the sufficient range of 30-100. Insufficient is 10-29 and deficient is <10. My lowest lab result was 7. My highest was 34. I live in the north, so that is a contributing factor; but, I make up for it with my diet, which is high in Vitamin D. As it is a fat soluble Vitamin, you should take it with a fatty meal, to help it metabolize.
My A1cs are good to moderate to no-so-good. Good being 4.8 - 5.2, moderate being 6 - 7, and not-so-good 8 - 13 (at diagnosis). I feel better on this current Vitamin D therapy of twice a week 50,000 iu and daily 2,000 iu.
The following is a tribute to Frank Garland, PhD who was a Vitamin D (and cancer) researcher.
His Studies Shed Light on Link Between Vitamin D and Cancer
UC San Diego Professor of Family and Preventive Medicine, Frank Garland, PhD, died Tuesday, August 17 in La Jolla, California, after a year-long battle with illness. Garland was also Technical Director of the Naval Health Research Center.
A San Diego native, Garland was born on June 20, 1950. He received his PhD degree in epidemiology from the Johns Hopkins University and began a distinguished career in vitamin D research for cancer and disease prevention.
Video Frank Garland, PhD - Vitamin D: Can Diabetes be Prevented with Vitamin D (47 minutes) Very Informative!
Also, here are some additional links. The most intriquing, to me, is the study in Finland, where as they reduced the amount of recommend Vitamin D (to US standards), the incidence of Type 1 Diabetes rose. (Dr. Garland, if I recall, references this Finish study in the video above)
I’m looking for a link I had on the US Military and Vitamin D. I’ll add that once I find it.
Putting the Pieces Together Health Diabetes Type 1 Prevention and Vitamin D
http://www.ppt-health.com/disease-protocols/diabetes-protocol/diabetes-1-and-vitamin-d/
Another: Hormone Health Network from the Endocrine Society Vitamin D
Another: Oregon State University Micronutrient Information Center Vitamin D