Not Just One Adaptation in Play
The ability to vomit weapons grade acid involves a highly complex and specific series of mutations in unrelated gene groups. First is the Acid Production, which would require some pretty specific tinkering with the genes related to the development of the stomach's Parietal Cells that are responsible for producing acid. Next it has to be retained without effecting the rest of the digestive process or and also without dissolving the person's abdominal organs themselves. The entire digestive tract would need completely different biochemistry that would most likely radically change the diet, metabolism and nutritional requirements for the now no longer human body. Next the throat, teeth and mouth would likewise have to be somehow altered to not be dissolved by this industrial strength acid. In short, from the lips to the anus your (no longer) human would need to be totally altered just to not be dissolved by it's own acids.
Next you need a method of projecting this acid, which would require a different structure of the abdominal muscles and the abdominal cavity in order to achieve any sort of projectile vomit beyond the meter or so that humans are presently capable of. Assuming all of this restructuring and adaptation hasn't altered the volume of the stomach the average adult has about a 1 liter capacity, however this is the amount held when full, the stomach is not usually filled up to 1 liter capacity with acid. Our digestive system typically ranges from containing .4 to 3 ounces of acid within its 1 liter volume. Not very much actual acid is needed for a human to digest what we do, so assuming your vomitous freaks are all somehow packing a liter of acid they would need to purge their stomach prior to eating. Probably they would pre-digest their food prior to ingesting it in much the same manner as a fly. They would vomit upon whatever they plan to eat, and then drink down the resulting slurry.
Forget Human, Its just Humanoid
The biggest limitation to having a human being able to safely produce weapons grade corrosives and then project them a significant distance to mean anything is the whole human part of the equation. For instance, our mouths are designed for things to go in, not for projecting them out. Our teeth are highly susceptible to decay due to even mild acidic compounds and our esophagus is not really meant for things to go back up it except for on rare occasions when the damage done to the body by vomiting is less than that from failing to purge something toxic or infected. Bulimic individuals often suffer from assorted throat cancers, rapid tooth decay, and damage to their respiratory system. The best way to adapt this particular function is for your creature to simply be humanoid, because the myriad of things that must happen for this to be feasible mean that by the time its all said and done the organism in question is no longer part of the human species.