Hydrolyzed vegetable protein (HPV)is vegetable protein, principally soya, hydrolyzed such that it is split into its constituent amino acids.  It is a very strong stimulator of the umami taste receptors.  It is used in many food products, of note are cheap gravy granules which are largely potato starch with a little HPV, and cheap soy sauce which is largely HPV and burnt wheat in water.  As I understand it these would include milligrams of HPV per pack.  I think it has similarities to monosodium glutamate in use and quantities. One can purchase it in kilogram/tonne quantities in China, in the price range $5 - $20 per Kg. Is it possible to buy HPV in consumer quantities without having to have it mixed with something else, such as potato starch or burnt wheat? 
“Bragg” brand liquid amino acids (which I’ve seen at most health food stores I’ve gone to) is hydrolysed soy protein and water; so that’s “mixed with something else” but presumably you were going to involve liquids in your food anyway. 