When I see a kitty with chronic vomiting, the first thing that comes to mind is inflammatory bowel disease. This can be related to a food allergy, so a diet change to a hydrolyzed protein diet or limited ingredient diet are the first things I try. The most common food allergens in cats are chicken and fish. Some prescription diets that are great places to start include Hill’s Z/D, Royal Canin HP and Purina HA.
IBD can also be an immune problem where the immune system is clogging up the intestines with white blood cells leading to maldigestion, diarrhea and weight loss. The best treatment for this usually steroids such as prednisolone and can also include buspirone.
Another possible cause of weight loss and low appetite would be pancreas problems (either inflammation- pancreatitis or lack of production of digestive enzymes- exocrine pancreas insuffiency). The best way to diagnose this is with a "GI Profile" blood work (both Antech and Idexx have this profile as well as the GI Lab at Texas A&M University).
The last cause that comes to mind would be intestinal lymphoma. This can appear very similar to inflammatory bowel disease and biopsies can be needed to differentiate the two diseases.
Abdominal ultrasound is a great diagnostic test for your kitty. Your veterinarian can measure the thickness of the intestines as well as look for inflammation around the pancreas. I would also ask about the "GI Profile" as this can also aid in the diagnosis of intestinal disease. The last step to consider would be an abdominal exploratory surgery. This is where your veterinarian would anesthetize your cat, open up their abdomen, visually examine all of the organs and collect biopsy samples for analysis. This is the gold-standard for diagnosing IBD vs intestinal lymphoma.
Does this help? What other questions do you have?