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The most likely cause of chronic vomiting in younger cats, is IBD (inflammatory bowel disease). That would be my guess here. The hard part, is diagnosing it. We have to get biopsy samples of the intestines to diagnose IBD. This is usually done at surgery. Endoscopy can be used, but surgical biopsies are more likely to yield a diagnosis.
Treatment is then with dietary management, steroids, antibiotics as needed and sometimes other immuno-suppressant meds. Here is a nice link on the condition to read over:
I hope this helps, let me know if you have any other questions.
Whoops, sorry about that.
Here it is: