What I am going to suggest, jak2002003 already suggested and you say you've already tried it, with no improvement. My 6.5 year old Golden was vomiting, a little bit frequent when I first got him at 3.5 years. As it turned out, I had been giving him too much dry food all at once. I cut it back to two, half-size feedings and never again vomited.
I wish you success in solving this issue.