Condolences. Read the site blood sugar 101 and learn about the part that its NOT YOUR FAULT.
Truly, its not your fault. And another word for prediabetes is insulin resistance. I think if you were already on Metformin, you were doing your best, and so was your doctor.
As a former vegetarian and vegan, and not knowing your reasons for wanting to be vegetarian, I want to support your wishes, but your health is most important.
There is a diabetes reversal diet that is vegetarian I believe, and you may want to try that. For my part, I prefer the higher fat, lower carb way, but you have to find whats right for you.
GOod luck and stay tuned in on the chat
zen and the art of weight loss, finding the true path of en-lighten-ment