I always like to play it safe and take my cats to the vet rather than give them something I'm not sure about. It could be something as simple as Simon may be eating too fast. I have a cat, Creamsicle, who scarfs down her food as though it's her last meal and sometimes she doesn't even make it away from the food dish and up it comes. My vet told me to place a large rock in the middle of the food dish, which will force her to eat around it, thus she'll be eating more slowly. If you think Simon eats too fast, give that tip a whirl. Also, if his vomit is mainly food chunks, then that may be what it is. If, however, it's mainly liquid and/or bile, then he needs to get checked by your vet. Bile means the liver isn't functioning properly. My vet's rule is that if my cat throws up once, don't be alarmed. If she throws up twice (in one day), I'm to call him. Please keep us posted.