For whatever reason that your couch smells of vomit, the need to get rid of that odor is certainly urgent. The best thing to do is NOT let the vomit dry and stick to the couch. As soon as the incident is over, clean it up!
Mop up the vomit with soap and water – the stronger smelling the soap, the better. You can even try soaps that are made to remove the odor of pets. These should work against the odor of vomit as well. Another trick is to use vinegar. Mix one part white vinegar with two parts water and use this mixture to get rid of the smell.