Why isn't vomit corrosive?
First of all, it is corrosive. People which are bulimic often get problems with their teeth getting damaged by the permanent exposure to the acidic stomach fluids. See the paper for more information:


Bulimia and tooth erosion


Then it is a matter of concentration. According to Wikipedia, the concentration of the acid in the stomach is around 0.5% which is a pretty low concentration. Additionally the acid gets diluted when you eat or drink things. And last but not least not all things are very susceptible to a fast degradation by acids. Thing tend also not to disappear suddenly, if they come in contact with acids.