A dog can vomit for many reasons. Sometimes it is a temporary irritation of the stomach after eating something unsuitable, but vomiting can also be seen with infection, inflammation, poisoning, disease in other organs or when something has got stuck in the gastrointestinal tract. A single episode in a bright dog therefore does not have to mean that something serious is wrong.
Look at the whole picture. Can your dog drink and keep water down? Does it want to eat? Is it alert and behaving as usual? Also note whether the vomit contains blood, whether the belly seems tense or tender, and whether your dog may have got hold of medicines, poisonous substances, toys, bones or other objects. With recurring vomiting, the vet may need to examine your dog and sometimes add blood tests or imaging to find the cause.
Seek care sooner if your dog vomits repeatedly, becomes lethargic, is clearly in pain, cannot keep water down or if there is blood in the vomit. If you suspect poisoning, or that your dog has swallowed an object, contact a vet right away.