Dreams mostly serve as a way for your subconscious to process everything. Dreams are just simulations. Typically, the dreams you remember are in a state of fearlessness. We fall into REM, and we are paralyzed, and our body turns off the ability to fear so we can face our fears in mental simulations. This is why REM sleep is so important for emotional well being. So throwing up in a dream is really just a way to help the dreamer accept the fact that they can throw up. Maybe they need to get something out of their body, but the dream (if you happen to remember it) is showing them how to do just that. Even if like you say, it symbolizes something that might not be food at all.