(The first Y in this name is a vowel and is pronounced like Y in 'my'. SHA ia pronounced like one between s in sun and sh in shut. The second Y is a consonant and is pronounced like y in 'you'. N is pronounced lika n in 'nut'. )
Vyshampaayana was vyaasa's disciple. He learnt one Veda ( Yajur-veda)and the Mahabhaaratahm among others, from his mentor. He was told by Vyaasa to narrate the story of the Mahaabhaaratham to the king Janamaejaya after the latter dropped his serpent-sacrifice in the middle of it.
Vyshampaayana was the preceptor of the great sage Yaagnavalkya to whom he taught the vedas. But learning that his disciple gave way to pride and arrogance in his heart, he punished his pupil by ordering him to discharge all that he learnt. Yaagnavalkya obeyed the orders and vomited out his knowledge through his month. Some birds ( thiththiri birds) ate thevomiting and thus that branch of veda became known as 'thyththireeya ' (= of thiththiris) branch.
While relating the story of Mahaabhaaratham to Janamejaya, Vyshampaayana clarified many doubts of the king. In the end, he elaborated about the period of status of the war-killed warriors in the heaven and their final attainments.
Vyshampaayana was a great sage, at any rate.