He was a pupil of Vyasa, from whom he learned the Jaya, the original 8,800 verses of the Mahabharata. He later expanded the Jaya to 24,000 verses under the name Bharata, which he recited to King Janamejaya at his sarpa satra (snake sacrifice). The Harivamsa is also said to have been recited by him. The full 100,000 verses of the Mahabharata was not complete until several centuries later.
^Raychaudhuri, H.C. (1972). Political History of Ancient India: From the Accession of Parikshit to the Extinction of the Gupta Dynasty, Calcutta: University of Calcutta, p.38
Janmajeya who is the son of Parikshan, who is the son of Abhimanya, who is the son of Arjuna. Janmejaya humbly asked Sri Vedavyasa Maharshi. How did this great war between kauravas and Pandavas happen in spite of great people (who knows dharma) like you, Bhismacharya etc., Then Vedavyasa Maharshi looked at his pupil Vaisampayana and asked to tell the story. Then vaisampayana started narrating the story. The story was narrated initially by sauthi until vaisampayana came in. There was not a single sloka regarding Vaisampayana in Mahabharat.
Source: From the Pravachanam of Sri Chaganti Koteswara rao garu