The timing of vomiting food, along with the nature of the vomitus, can be a good indicator of where the problem lies. Vomiting food immediately after eating would point to a stimulus or problem in the esophagus. Acid reflux might be indicated, though there could also be other causes. One would expect the vomitus to contain food that was virtually as-eaten, possibly with clear or frothy digestive juices.
Vomiting a short interval after eating (1/2 - 1 hr) of undigested or partially digested food would point at the next position, the stomach. I don't know all of the possible causes, but would logically expect it to be related to irritation or overacidity. One source of irritation might be antibiotics or other meds. In some cases adjusting the dosage or switching to an alternate medication might solve the problem. In Coco's case, a standard dose of Clavamox (calculated for a cat up to 12 lbs) would irritate her stomach, but she could tolerate a dose calculated for her actual weight of 5.3 lbs. She was also ok when Amoxycillin was substituted for Clavamox. Ferrous sulfate as an iron supplement can be very corrosive to the stomach, whereas more organically-derived sources are quite innocuous.
Vomiting after a longer interval would normally originate further down, in the intestine. Food would have been digested to a uniform brown liquid and would contain bile. Without going into (or necessarily knowing) all of the possible causes, I'd venture a couple of guesses for lower-tract vomiting. One would be intestinal tract bacterial infection, needing antibiotics. Another would be depletion of intestinal flora (beneficial digestive bacteria), if a long course of antibiotics has been given. This might be helped by acidophilus and other cultures, either in the form of additives or yoghurt, which has several beneficial cultures (the least pasteurized probably better, though any will help).
Other more serious possibilities that might need to be pursued (if neither of the above seems to apply) would be IBD (inflammatory bowel disease, probably prednisone) or neoplasia. I don't have any personal experience with either, but my impression is that more invasive means might be necessary to pin either of these down, so you'd probably want to rule out (and possibly attempt to treat) the other possibilities first.
Vomiting of undigested food after an interval of much more than two hours might indicate that the movement of food through the GI tract is not occurring at a normal rate. If this is the case, your vet may prescribe Reglan, a powerful drug which stimulates the rhythmic contractions necessary to keep things moving in the right direction. My impression is that it's so strong that it's virtually impossible for a cat receiving it to vomit. I don't know if the same clinical symptoms might be caused by a blockage, however, so additional tests such as x-rays or at least an expert abdominal exam may be necessary to confirm a diagnosis.
Often a cat will make a characteristic vocalization prior to vomiting. If you get such a warning, try and catch him and put him over a newspaper when he is going to throw up. Stroke him rhytmically from the shoulders back, speaking in a reassuring voice. If it's just heartburn/acid, it may help relax him and avoid the episode. Otherwise it may make the experience go more smoothly and less traumatically. It lets your kitty know that you care and want to help. If the newspaper is used successfully, it gives you a good background on which to inspect what came out, and to preserve a sample to show the vet if needed.
Repeated vomiting may be significant, and if it's a regular event, it would be good to keep track of. A simple log on a sheet of paper indicating time of day, estimated amount, character of liquid (color, opacity, froth, etc.), level of digestion of food, time since last meal or meds, etc. Just enough detail to help you remember it and describe it to the vet if need be. For instance, does it happen after eating, after sleeping, on hot days only, etc. Anything that could possibly be a factor, if written down, could help discover a trend.
I'm not normally an organization freak, and don't make lists for to-do's, shopping, etc. I am, however, a very strong advocate of logs to note down anything that might be indicative. WE are the primary caregivers for our furry family members, and we should keep AT LEAST as good a chart as the vets that they see (hopefully) so much less often. Having good records enables you to talk to your vet in detail. Your cat's chart is probably several (if not dozens) of pages long, and no vet can possibly remember it all, or read it while you are on the phone. Your obvious involvement will inspire the vet to suggest more alternatives, rather than taking the easy road to euthanasia as an early recourse.