the 100% vegetarian diet. marked on almost all health food store packaged meats - sometimes even 'grass-fed' meats - is to assuage fears of animals being fed the remains of other animals, which have been publicly implicated in many health scares. It means very little to us, as vegetarian fed surely means grains, and possibly refined grain products and excesses of processed food production. I highly suspect by 'vegetable diet' they simply mean vegetarian diet.
as for corn being a vegetable, vegetable is just a made up distinction anyway so its possible to distort in such a way. Technically they are closer to being a fruit, but since it does not come from the reproductive part of the plant, and pours form its vegetative parts, its known as a caryopsis or dry/fruit i.e. grain. In many ways grains are closer to being a fruit/vegetable than seeds like quinoa, Amaranth etc..
vegetation including seeds and grains like non-gmo corn, are perfectly healthy for a chicken to eat IMO as long as it gets access to pasture/insects. chickens indeed eat grass and can/will eat vegetables, but suspect no chicken will be fed 100% vegetables without some sort of seed/grain or pasture, nor should they.http://www.lionsgrip.com/feedinstruc.html
here is the diet of a wild turkeyhttp://www.nhptv.org/natureworks/wildturkey.htm