I take Vitamin D3 in the winter because I'm of an age when I don't manufacture it properly, and I don't get out as much in the winter to get the sun on my skin. I take 5000IU, which I researched and am happy with, but it's well over the Recommended Daily Allowance, so do your own research.
I also take 300mg of magnesium every day, in two doses. This is supposed to be good for blood pressure, and mine is okay despite my weight, so it might be working! If you overdose on magnesium you get the runs, so you'll know :shock:
I don't take either of those specifically because I'm diabetic, but I have stopped taking large doses of Vitamin C in the early winter because Dr Richard Bernstein (The Diabetes Solution) says too much can raise bg levels.
A varied diet with plenty of vegetables and some fruit should see you okay. I believe you have to be more careful about getting enough iron on a vegetarian diet.
Viv