Well, Kudos for asking a question I dont see three times a night....
Your digestive tract from one end (the mouth) to the other is about 33 feet long....digested food technically becomes feces about 6 feet from the end opposite your mouth....I'm afraid that its just not possible to vomit feces - unless (ahem) you put them in your mouth in the first place.
About the only thing you can vomit is your stomach contents. You have several sphincters - basically, elastic bands that prevent anything from getting in or out. You have one at the top of the stomach (food can only get in/out while you are in the act of swallowing/vomiting), a one way valve at the bottom of the stomach - so once food is in the small intestine, the only way its coming out is through door number 2) - and then the last one is the anal sphincter.....and well, you probably know enough about that one anyhow.
Edit: I looked up what Alpla wrote about bowel obstruction - apparently I was miseducated, and you most certainly can vomit feces in the case of complete bowel obstruction.
Give the man a thumbs up!