What exactly has the vet checked for? Allergies? Thyroid? Fleas?
My uncle's cat lost all the hair where his collar was...and it turned out he was diabetic.
If he is losing weight an vomiting, and is obviously not healthy (the cancer is certainly not contributing to his health!), he may be unable to process his food well enough to derive nutrients from it...in that case digestive enzymes/probiotics might help, as well as a diet change...
I'm sorry I could not be of more help and I hope you figure out what's wrong!