I can answer this one!!!!
I have a dog
who has thrown up
12 hours for the past 2 years. He's had every test in the book, blood, urine, endoscopies of both ends, and more blood tests. Repeatedly.
There is no reason for it that we know of. Our vet calls these dogs "Puker Dogs", and said that work ups often show no discernible cause. Of course, you HAVE to do the work ups to be sure its not something serious, first of all, as you are doing.