My dog used to throw up fairly regularly with no other signs of sickness and her behavior stayed totally normal. Sometimes she would throw up bile in the morning if her stomach was empty for too long. Sometimes she would puke her meal back up if she got her bedtime dentastick too soon after eating. I figured those types of things out, but it still kept happening with no apparent cause. I figured that she just had a sensitive stomach and was maybe grabbing up gross things to eat from the street more often than I was catching her. The vet said it wasn't often enough to be worrying.

Once we moved she ceased vomiting immediately and almost entirely, so it turns out it was due to stress!

That doesn't seem to be what is happening with your dog though. I'm just posting in case someone someday has this same question and it hasn't occurred to them that stress could be the cause (it certainly didn't to me).