The entire Marmite range is vegan and it (the British version) has 0.74 mcg of B12 per teaspoon.
As to the original OP if metformin made me feel constantly ill and as it does not necessarily have a huge effect on blood sugars I would discuss it with my doctor and ask for an alternative which there are a number of I believe then I would bin it, but that's just what I would do what you do is entirely your decision no one else can decide for you not doctor's not DN's or anyone else.