Your plan may work if you free range on pristine pasture/forage in warmer weather.
Or if not free ranging on good forage it will work if you just want a few birds for a while with no regard to optimal production or health.
However, a vegetarian based diet will quickly cause nutritional deficiencies in omnivores.
You will likely see retarded growth, reduced egg production and smaller egg sizes - if they lay at all.
At least 10 essential amino acids that must be provided in poultry diets are: lysine, threonine, tryptophan, methionine, cystine, isoleucine, histidine, valine, arginine, tyrosine and phenylalanine. A fruit, vegetable and fodder diet will have some limiting amino acids and most likely lysine, methionine, tryptophan and threonine.
This is especially true in growing animals.
There are likely some vitamins and minerals that will be missing as well.
There has been well over 100 years of exhaustive research into the nutrients chickens are known to need and exactly how much of each. Those are in the bag of feed. Feed manufacturers do analysis of their feeds to ascertain that the feed in fact contains all those nutrients, fats and energy in the proper ratios.
It isn't just as simple as that. The digestibility of the amino acids and the true metabolizable energy in the feed need to be considered.
You can read up on vitamin and protein deficiencies in poultry here.
http://www.merckvetmanual.com/poult...ltry/vitamin-deficiencies-in-poultry#v3347975
http://www.merckvetmanual.com/poult...mino-acid,-and-energy-deficiencies-in-poultry
If one wants healthy poultry that are optimally productive, it just isn't cost effective to avoid manufactured feed.
If you simply must make up your own food for your birds, this feed calculator may help.
http://modernsettlers.com/poultryfeed/#