Occasional vomiting can be a difficult case. There are quite a few dogs, especially younger ones, that will vomit a bit of froth like you describe every so often. With no other problems than this, it is difficult to throw a lot of resources into trying to figure out exactly why this happens. While dietary indiscretion usually is seen by the vet when more serious gastroenteritis results, there is clearly a whole spectrum of ill effects. Vomiting is also possible from neurologic or metabolic disorders.
Vomiting can also be a sign of serious disease.
If there are any other problems noted you should see your vet about this for more followup. So I cannot provide diagnoses, or really tell you to ignore or treat a given problem. I hope the information provided guides you in making good health care decisions for your pets.