All, no effect, Why?
Microwave, i.e. Higher frequency than radio but lower than infra-red waves, is just a way of heating mostly the water in food, from below the surface instead of just from outside in.
Vitamins can be degraded at high temps, but higher temps, at the surface of foods, is a larger issue for usual surface heat cooking methods. See: http://en.Wikipedia.Org/wiki/electromagnetic_radiation.