If there is mostly undigested food in the vomit, then the former is probably the culprit. Try putting down smaller portions more often for a while. For the latter, the most digestable generally available foods are supposed to be chicken and rice. You can check with your vet about even more digestable special diets. Some cats are said to have trouble with lamb or beef, which are very common even if not listed as the flavor on the front.
Monitor is weight and activity level.