The definition of vomit is: to eject the contents of the stomach through the mouth (usually involuntary). A secondary definition is to eject or discharge in a gush.
Technically I suppose anything can spew out in a gush, if you kick an anthill over they may vomit from it looking to defend their home; however, I suggest you mean the first definition.
In order for an insect to vomit they must have both a mouth and a stomach, which they do have, at least for the most part. The definition also claimed that vomiting was usually involuntary, I don't think insect would ever vomit due to the flu, but a great many of them vomit on purpose. Honeybees for example do a great deal of vomiting. When a bee lands on a flower it eats the flowers nectar. While inside the bee the nectar loses its moisture, and by the time it gets back to the hive to deposit the nectar it has turned into honey. The folks at this site may help you learn more about bees, or at least they may help you not be disgusted with the details from the answer to your question. For more answers on honey bees check this link out:
http://www.beefolks.com/shopcontent.asp?type=faq2
Other insects will vomit for other reasons as well, a housefly vomits out a secretion that dissolves any potential food on a surface, then it sucks it back up, without vomit houseflies would not be able to east at all. So to definitively answer you question, yes insects can vomit, but it a little different way than we do.
