You seem to be very well educated in this topic
To my mind vitamins are useful when you are living a very busy life and when you have no opportunity to take much care of your diet, cos I know it from my own experience if you take calcium pills they work inspite that they are synthetic, but at the same time I use synthetic vitamins only when I have no opportunity to take natural food and when it is winter. Also I've read recently that if you're smoking taking some synthetic vitamins may cause cancer cells development. And the last thing I know is that if you have clean body (I mean from inside), e.g. after fasting your organism is able to assimilate much more vitamins from your food