More details would help. Did he hit his head? If not, what part of his body did he hit, and on what?
The proximity you describe is worrisome. Without more details it's hard to recommend anything except that you call your vet.
Head injury commonly causes vomiting. Without external signs of trauma, eg lacerations or swelling on his head, a single episode of vomiting without any other symptoms is probably ok, seeing as he didn't lose consciousness at the time. Symptoms of concern would include being dazed, wobbly, or disoriented. Seizure, decreased level of consciousness, bleeding from the ears or nose, and not holding his head correctly are very serious symptoms requiring immediate treatment.
If he didn't strike his head, look for trouble concerning what he did hit. Though not likely to be associated with vomiting, he could have bruised some ribs (watch for breathing abnormalities) or hurt some other part of his body. Run your hands over his entire body; if he reacts with pain, limps, or has other symptoms, call your vet.
Types of Head Injuries in Dogs
Trauma to the Chest in Dogs