Chronic vomiting and weight loss in a cat of this age can be caused by several different conditions. On the list would be:
Inflammatory Bowel Disease
Gastrointestinal Obstruction (foreign body, pyloric hypertrophy, intussusception)
Endocrine diseases (Diabetes, hypercalcemia, hyperthyroidism)
Central Nervous System Disease (brain tumors, infection, inflammation, head trauma)
As you can see, the list is really quite long. Chronic vomiting (2-3 weeks of regular vomiting) with or without other systemic signs usually requires a systematic and extensive diagnostic plan including a CBC, chemistry panel, T4, urinalysis and abdominal radiographs. Some diseases can be diagnosed directly from the baseline bloodwork like diabetes, liver disease or kidney disease. Other problmes require further blood tests for diagnosis (like chronic pancreatitis). Depending on what the findings are from there, abdominal ultrasound and/or endoscopy (use of a long camera put inside the GI tract) can be used to further define what the problem is. Prognosis and treatment is really going to depend on what the ultimate definitive diagnosis is.
For a cat with this type of history, the number one on the list would be inflammatory bowel disease. Although we tend to think of this as a diarrheal disease in people, in cats it is primarily a vomiting problem. The next most likely would be diabetes, kidney disease, hyperthyroidism and cancer. I am attaching some client information handouts that I use in my practice that discuss these problems in more detail. I hope that you find them useful.IBDChronic VomitingWeight loss
I hope that this information is of some help to you, and I wish you the best of luck with your cat. Please let me know if I can be of further assistance.