Well, a BCAA supplement on it's own is definitely not a "naturally occuring" product. BCAAs (leucine, isoleucine, valine) are found in foods with protein. BCAA supps can be helpful for someone interested in preserving/gaining muscle mass.
Nu-World puts out some Amaranth cereals that have a good amount of protein. Anyone try them? See here: http://www.nuworldamaranth.com/cart/products.asp?grouping_id=2
Yeah - you have to check with the company to see if its vegan. Some are derived from dairy. These brands have a vegan BCAA product: Scivation Xtend NOW Arizona Nutritionals (Blox) Optimum Nutrition