| If he is vomiting grass and twigs, in a clear or yellow fluid, then he might be just rejecting some fibrous material that he ate.
If he is vomiting his food, and if he does it more than once, he might be sick.
Did he have diarhhea only once? Have his stools been normal after the one incident?
Sometimes a dog can have a one-time event, which does not mean much, except that he had an upset stomach. But prolonge diarhhea or vomiting can mean illness of many different kinds, so if he is not straightened out very soon, take him to the vet.
How is his appetite? Was the diarhhea bloody? If he is eating well, it is a positive sign. I have no medical credentials. This answer is strictly from 35 years of observing symptoms of my own dogs. So, it is important to get him to the vet if he is not normal soon. | 
