As silkiechicken said, chickens are omnivores and need meat protein in their diets.  Not sure what the source is in commercial feed.  I'd think it would depend on the producer.  That being said, I wouldn't doubt that some producers use chicken in their chicken food.   Theoretically, it should be listed in the ingredients.  Of course, I've found that my hens are not unwilling to pick at one of their deceased compadres when a predator has gotten into the coop.  
		
		
	
	
AND, even if you find a vegetarian feed, I'd bet all the money in the world that unless you somehow rid the area around your coop of all animal life, your chickens will never have a completely vegetarian diet.  Mine love to go around a eat slugs on a dewy morning.  Japanese beetles, I've found, are a treat, and just about anything with 6 or 8 legs (with the exception of some kinds of ants) had better say its prayers before trying to cross my coop.  
On interesting side note.  I had a conversation a few months back with someone here about the eating habits of some animals.  They were saying that dogs were strictly carnivores in nature.  I was saying they were, like chickens, humans, and bears, omnivores.  Anyway, I was poking about some fox scat out in the woods last week (it's amazing what you can find out by poking through other peoples' sh*t  
 ) and found choke cherry pits (just ripening around here) among the small bones, exoskeletons of bugs and other unidentifiable ick.  Hmmm, last I heard, cherries were not meat.  Just food for thought.
Mark