There is a medication called Cerenia, which is highly effective for vomitiing in dogs. It is also expensive.
Beyond that, I have a doberman who has been vomiting
now for more than a year and a half. He's had every test known to the veterinary world, with
out any abnormalities.
My dog vomits only when his stomach is very empty, though. The Cerenia really, really helps. He's been on it for about 3 months now.