Um...you know how vampires can eat if they for some reason want to,right...?

Vampires can eat food, but according to SM's books, they have to "cough" it up later since their bodies won't digest it. Plus, it tastes like dirt, at least to Edward.

Well, as their digestive system doesn't work (any of their systems doesn't work!) they can't wait for the food to come out the right way... So they have to vomit it... Gross, but that's the way...

Hope this helps! =]