Vomiting alone is not always a reason to take your dog to the vet.  However, if it has vomited more than once in a day or more than twice in two days, you should definitely have your dog checked out.  Other causes for concern with vomiting which warrant a trip to the veterinarian are if there are other signs associated with it.  For example, if your dog is acting more tired than normal, or is not able to hold food down, or acting in pain.  If you would like more information about when to take your dog to the veterian when there is vomiting, check out this article here:  https://www.vetbabble.com/dogs/health-dogs/vomiting-dogs/