It seems to me that you could only get in trouble if the veggies were all they were all they had to eat. And, that would be especially true if the weather is cold.
Most vegetables, certainly of the green leafy types, have very few calories. And, nearly all veggies have lots of fiber that is difficult for chickens to digest. If the chickens are in the cold, these foods won't help them much in staying warm.
If the birds have a free-choice high-calorie feed, they may not eat much in the way of green veggies during cold weather anyway. They will tend to fill up on their feed. At least, this is true with my hens. I try to give a little higher calorie treats to them when it is cold.
Steve