You will notice when you read the side of the box that it is not made out of 'rice' and nothing else. There are a variety of brands made with a variety of ingredients, any of which she may be reacting to.
But vomiting is not the 'normal' reaction to food allergies. It seems much more likely that you have inadvertently purchased a box of contaminated cereal -- or that it has become contaminated at your house somehow.
Rice cereal was created for babies when they were dying of malnutrition and kidney failure as a result of dangerously inappropriate 'formula' which was badly made to replace breastmilk. Formulas today are far closer (although still a long, long way off) to real human milk, so are safer and the addition of other modified foods for infants is totally unnecessary, from cereal right through 'toddler' jarred foods.
She can go directly from formula to regular human food, while following the introduction cautions (no honey under a year, avoid egg whites until about a year, etc.), not a piece of information you'll get from the folks who sell the jars, though...