Would vitaminb12 therapy be good for diabetics?

Not unless deficient: Extra vitamins of any sort not helpful unless there is a deficiency or during pregnancy when folate (folic acid) is important.

Not really.: The B vitamins in general allow us to process carbohydrates more efficiently, but there's no direct link between B12 deficiency and diabetes as far as cause and effect. It's doubtful that treating diabetes with B12 will be dramatically beneficial.