Sanjaya acknowledges the mercy of Vyasa, by whose mercy he could hear the master of mysticism, Krishna
One has to understand Krishna not directly, but through the medium of the spiritual master
Spiritual master is the transparent via medium and experience is still direct
bKrishna as the master of all mysticism
Many mystics and yogis are there all over the world, but Krishna is the master of all yoga systems
Topmost yogi according to Krishna’s instruction: one who surrenders unto Krishna (Bg. 6.47)
cVyasadeva is as bona-fide as Arjuna
Because he comes in the disciplic succession and Sanjaya is the direct disciple of Vyasa
Vyasa is a disciple of Narada Muni who is a direct disciple of Krishna
dSanjaya could see and hear Krishna directly because
He is a direct disciple of Vyasa
Also senses are purified, by the grace of Vyasa
eDisciplic succession is must
Without disciplic succession one cannot hear Krishna and his knowledge (especially regarding Bhagavad-gita) is always imperfect
fSpiritual master is the representative of Vyasadeva
Therefore birthday of spiritual master is celebrated as Vyasa-puja