Scrape the vomit off of the upholstery with a plastic spoon. Discard the spoon in a trashcan.
Fill a small container with 1/2 pint cool water and 1 tsp. of an enzyme detergent. These detergents are designed for pet accidents; you can find them at pet stores. Mix the contents together with a spoon.
Dampen a clean, white cloth in the mixture. Blot, not rub, the vomit with the cloth. Begin at the outer edges of the vomit stain and blot with the cloth, moving inward.
Dampen a clean cloth with cool water and blot the upholstery to remove the cleaning solution.
Mix 3/4 cup water with 1 tbsp. ammonia.
Dampen a clean cloth in the mixture and blot the upholstery in the same manner as before. The ammonia mixture neutralizes the acids naturally found in the stomach.
Blot the mixture off of the upholstery with a clean cloth dampened with cool water.
Mix 2/3 cup cool water with 1/3 cup white vinegar.
Dip a clean cloth in the mixture and blot the area of the upholstery. Vinegar is an all-natural deodorizer that will remove odors from a variety of surfaces.
Let the mixture air dry on the upholstery. As the vinegar dries, its odor will dissipate, taking the vomit smell with it.