I'm confused.

I've always been under the general impression that Vitamin C was a good thing although it's overhyped and it's difficult at times to separate hype from fact when talking about Vitamin C, particularly when talking about mega-doses of Vitamin C.

The Linus Pauling Institute (the main source of Vitamin C hype) says

Cardiovascular diseases (heart disease and stroke) are the leading cause of death in individuals with diabetes. Evidence that diabetes is a condition of increased oxidative stress led to the hypothesis that higher intakes of antioxidant nutrients could help decrease cardiovascular disease risk in diabetic individuals. In support of this hypothesis, a 16-year study of 85,000 women, 2% of whom were diabetic, found that vitamin C supplement use (400 mg/day or more) was associated with significant reductions in the risk of fatal and nonfatal coronary heart disease in the entire cohort as well as those with diabetes

That looks good. But then they follow up with this

In contrast, a 15-year study of postmenopausal women found that diabetic women who reported taking at least 300 mg/day of vitamin C from supplements when the study began were at significantly higher risk of death from coronary heart disease and stroke than those who did not take vitamin C supplements

That looks bad, at least for postmenopausal women with diabetes. It causes them to conclude

The significance of these findings is not entirely clear, but they suggest that there may be a subpopulation of people with diabetes who will benefit from antioxidant therapy while others may not benefit or could actually be harmed.

and

Since vitamin C intake from foods was not associated with increased mortality from cardiovascular disease, there is no reason to limit the intake of vitamin C-rich fruits and vegetables.

What might those Vitamin C-rich fruits and vegetables be? The most hyped is oranges, which is also pretty high in sugar. So that one might not be the best choice for diabetics. Two good choices are cabbage and peppers, especially red peppers. The dark green leaves from Chinese cabbages are particually good choices.