Sorry to hear about your cat, Mamav...
The poison's source could be chemicals used to kill gophers on the golf course, various plants and shrubs, household chemicals, antifreeze, and even poisons deliberately placed by "bad guys"; particularly warfarin, the active ingredient in rat poisons.
You did well by bringing the cat to the Vet right away, and being that the cat was vomiting, is definitely a good sign, being that some of the toxin is being expelled... I imagine that he checked for intestinal bleeding, took a sample for the animal's gastric contents, examined the cat's coat for contact type poisoning, took a blood sample, went over general signs, etc.
Did the doctor give the cat some activated charcoal via a stomach tube to slow down gut absorption of the poison? Usually 1 part of activated charcoal to 6 parts of cold water is used... After 30 minutes, he might have given the cat a dose of Milk of Magnesia to try and expel even more of the poison...
Did he tell you that the cat had neurological signs and symptoms of poisoning? This is not good, but being that you didn't mention this in your post, the cat probably does not have these. If the animal did, the Vet would probably want to keep the animal at his clinic for other therapies and observation.
Keep fresh water near where your cat is recuperating, and make sure it is kept in a warm, dimly lit area, without distracting noises... I trust that your cat will recover soon, and go for some food by the time your read my post...
