Let's leave the subject of cooking off to one side and since the subject of vitamins is on topic just look at that in a non-cooking sort of way:
I guess chickens are lactose intolerance just as I am. Milk contains all these vitamins beneficial to critters. All that A, B, D, E, and K, and supplemented vitamin D aren't going to do either me or the birds one tiny bit of good if everything passes thru as fast as "grass thru a goose."
Soy and other legumes such as kidney beans and lentils have substances that interfere with digestion. The hens can "eat" them raw but that isn't going to do them much good. All those B vitamins and iron available in those legumes and even the carbs and proteins probably won't add up to a "hill of beans" for the nutrition of the birds.
Mixing my metaphors and heck, I don't know whether I'm on topic or off anymore. Probably, the original poster has lost all interest anyway.
Steve