I did take a general vitamin supplement when I was first diagnosed and made the decision to instantly go very low carb. I just wanted a bit of a back up while I was getting used to my new diet. And metformin made me very nauseous and I didn’t eat much for the first 3 months.
However I had some blood tests a few months ago and everything was fine, iron actually borderline a bit high. So since then I have cut it down to a couple a week until I use up the giant pot, and I do take vit D because I go whole weeks in the winter without setting foot outside.
If you feel the need I don’t think they do any harm, just choose a reputable brand and don’t go stupid on the dosage.