I too have an older cat nearly 12 yrs, he vomits too, sometimes he won't do it for a while ,then he just does it continually for a week or so, almost straight after eating, i have had my vet check it out and he has had blood work, they have not found anything,the vet told me some cats just vomit like that, she had one herself, did everything, changed to a special diet etc, and it never did any good. I have complete trust in my vets, have been going there for years and they are excellent.

I think the main concern is whether your cat is loosing weight or not, my cat never does, so i guess that is a healthy sign,but really if you are at all concerned and it sounds as if it is just a recent thing, then follow the other's advice here, maybe withdraw the food, could be allergy related, and seek another opinion, always pay's to if you are not satisfied with what you have been told, good luck and keep us posted.