Krishna knew the curse of Vasus. What can one say about him? He is an avatar of Vishnu himself and there is evidence where he tells Ganga not to mourn for Bhishma.
The puissant Krishna, hearing the goddess of the great river indulging in these lamentations consoled her with many soothing words. Krishna said, 'O amiable one, be comforted. Do not yield to grief, O thou of beautiful features! Without doubt, thy son has gone to the highest region of felicity! He was one of the Vasus of great energy. Through a curse, O thou of beautiful features, he had to take birth among men. It behoveth thee not to grieve for him. Agreeably to Kshatriya duties, he was slain by Dhananjaya on the field of battle while engaged in battle.
https://www.sacred-texts.com/hin/m13/m13b133.htm
Krishna tells this to Ganga when Bhishma went to the abode of Vasus on Uttarayana.
https://www.sacred-texts.com/hin/m01/m01068.htm
Vaisampayana said, "And from the curse of Vasishtha and the command also of Indra, the eight Vasus were born of Ganga by her husband Santanu. The youngest of them was Bhishma, the dispeller of the fears of the Kurus, gifted with great intelligence, conversant with the Vedas, the first speakers, and the thinner of the enemy's ranks."
Vaishampayana is the one who recites the narration of Mahabharata to Janamejaya at his snake sacrifice so both Janamejaya and Vaisampayana know that Bhishma was born because of the curse of the Vasus
And when he had been saluted, and was seated, Vyasa addressed him thus, 'O guide of the Ganas! be thou the writer of the Bharata which I have formed in my imagination, and which I am about to repeat."
"Ganesa, upon hearing this address, thus answered, 'I will become the writer of thy work, provided my pen do not for a moment cease writing." And Vyasa said unto that divinity, 'Wherever there be anything thou dost not comprehend, cease to continue writing.' Ganesa having signified his assent, by repeating the word Om! proceeded to write; and Vyasa began; and by way of diversion, he knit the knots of composition exceeding close; by doing which, he dictated this work according to his engagement.
Ganesha wrote the Mahabharata by listening the words spoken by Vyasa. Vyasa was the one who spoke and Ganesha listened and wrote epic also so both of them know that Bhishma was a portion of Vasus born on earth due to a curse.
Ganga drowned seven children who were portions of Vasus in a river and as she was about to drown the 8th kid, Santanu stopped and Ganga told him why she drowned the children.
But when the eighth child was born, and when his wife as before was about to throw it smilingly into the river, the king with a sorrowful countenance and desirous of saving it from destruction, addressed her and said, 'Kill it not! Who art thou and whose? Why dost thou kill thy own children? Murderess of thy sons, the load of thy sins is great!'"
Ganga explains
But according to our agreement, the period of my stay with thee is at an end. I am Ganga, the daughter of Jahnu. I am ever worshipped by the great sages; I have lived with thee so long for accomplishing the purposes of the celestials. The eight illustrious Vasus endued with great energy had, from Vashishtha's curse, to assume human forms. On earth, besides thee, there was none else to deserve the honour of being their begetter. There is no woman also on earth except one like me, a celestial of human form, to become their mother. I assumed a human form to bring them forth. Thou also, having become the father of the eight Vasus, hast acquired many regions of perennial bliss. It was also agreed between myself and the Vasus that I should free them from their human forms as soon as they would be born. I have thus freed them from the curse of the Rishi Apava.
So even Shantanu and Ganga knew about the curse and Vashishta himself was the one who cursed.
https://www.sacred-texts.com/hin/m12/m12a047.htm
"I have, however, resolved to enhance thy fame, O thou of great splendour! It is for this, O Bhishma, that I have just inspired thee with great intelligence. As long, O lord of earth, as the earth will last, so long will thy fame travel with undiminished lustre through all the world. Whatever, O Bhishma, thou wilt say unto the inquiring son of Pandu, will be regarded on earth to be as authoritative as the declarations of that Vedas."
So Krishna gives Bhishma a boon of knowing the Past, Present and Future.
https://www.sacred-texts.com/hin/m12/m12a047.htm
Krishna gives Bhishma a boon of knowing the Past, Present and Future to instruct Yudhisthira after war. If Krishna gave Bhishma the boon of past, present, and future, Bhishma would know the past also and if Bhishma knew the past, he would know why he was born in world of men.
"Learning by his Yoga prowess of the devotion of Bhishma, Madhava, otherwise called Hari, (entering his body) bestowed upon him heavenly knowledge compassing the Past, the Present, and the Future, and went away. "
This is during Shanti Parva where Krishna gives Bhishma that boon because Krishna wanted Bhishma to instruct Yudhisthira on statecraft and duties of king as he wanted his fame to be immortal on earth even though Krishna himself could have done it.
So Vashishta, Krishna, Ganga, Santanu, Janamejaya, Vaisampayana, Ganesha, and Vyasa and all other people who were present and listened to the first narration of Vaisampayana during the Sarpa Satra of Janamejaya, and Bhishma himself knew about why Bhishma was born and how he was a portion of Vasus.
I hope by reading my post, people know that Bhishma was born due to curse of Vasus and also know who all knew it.
