Take him to a vet asap! He may have had a stroke or he may have high blood pressure
which could leed to a stroke. Always take your pet to the vet right a way when you
notice something not right. Its better to be safe than sorry!!
We are not doctors, so all we can do is guess. What you describes could be a long list of conditions. I can add a few more in addition to the above post-diabetes, kidney disease, liver problems, etc.
The vet will need to run bloodwork, maybe take x-rays to see if something shows up. Maybe he'll add a few more tests depending on what the blood work results show. I'd definitely ask about diabetes and kidney disease, especially if you've noticed your cat drinking a lot more water lately.
Well reasons are many. Your cat may be losing balance because he probably had a stroke, viral disease or ear infections. Whatever is causing this as well as the vomiting and weight loss is potentially dangerous. You need to contact your vet ASAP. Your cat may look normal outwardly, but have very serious internal problems. Everything you mentioned is more than enough reason to take the cat to the vet. Don't wait any longer.