Parshas Vayechi begins with Yaakov asking his son Yosef to swear a specific kind of oath to him, that he will take him out of Egypt and bury him in the land of Israel. The only other time we see such an oath applied is when Avraham obligated Eliezer to do the same, to swear to him that he would not allow Yitzchak to intermarry, that he was to take a wife only from his own people and that he would not take Yitzchak out of Israel.
Both oaths were applied when Avraham and Yaakov were nearing their end. Of Avraham it says, VeAvraham Zaken Ba Bayamim. And Avraham was aged in years. Of Yaakov it says, VaYikrevu Yemei Yisrael Lamut. And the time had come for Yisrael to die. Both of the fathers of the Jewish people, when nearing their end, looked to preserve their legacy and their heritage. To unite the past with the future. And though both men were nearing their end, the oaths are applied in Parshas with the word Chai in their title.
Avraham causes Eliezer to swear to him in Chayei Sarah. Yaakov causes Yosef to swear to him in Vayechi. Though both parshas narrate the end of their lives, they are described using Chai, because through these oaths, they lived on.
The common denominator between both oaths was the integrity of the Jewish people and the land of Israel. Avraham calls on Eliezer to provide a Jewish future for his son. A future in which he would marry a wife from his own kinfolk, while still remaining in the land that G-d had given him. Yaakov asks Yosef to bring him back to that same land, to the resting place of his forefathers. Both men were not just serving their own needs, but making a definitive statement about the unity and continuity of the land and the people.
The Jewish people could not exist had Yitzchak intermarried or abandoned the land. And it could not exist if the Jews had come to think of Egypt as their own land. That is why Avraham and Yaakov both applied the oath to the Brit, with which G-d had sealed an eternal covenant with Avraham that He would make him into a family of nations. The covenant depended as much on Yitzchak not intermarrying and remaining in the land, and on Yaakov returning to Hevron accompanied by his children, as it did on the Brit itself.
And it is why Yaakov only blesses Yosef's children after the oath, and treats them as his own sons, after the oath was taken. Because only by showing that commitment to the land and the people, could Yosef be considered worthy enough to have his children become tribes of Israel.
But one question remains. Why did Yitzchak not apply a similar oath to Yaakov, when the latter was sent off to find a wife with Lavan? Yaakov was both leaving the land and going off into a situation in which he might follow his brother's example and marry inappropriately outside his parents' supervision.
There were three conditions that caused the oaths to be applied. The first condition was that the one swearing had to have emotional resistance to carrying it out. Eliezer wanted Yitzchak to marry his own daughter. Yosef wanted his father to be with him even in death, and worried about triggering Pharaoh's wrath. The second condition was that fulfilling the oath required overcoming physical resistance by getting permission from a third party. Eliezer had to get permission to take Rivka with him. Yosef had to get permission from Pharaoh to bury his father in the Maarat HaMachpelah. The third condition was that it would take a miracle to accomplish the oath. For Eliezer it was the miracle of the well. For Yosef, it was the miracle of the Caanani kings standing aside to allow the burial to proceed. The oath of the brit invoked G-d as a partner in seeing that the oath was fully carried out.
But Yaakov had no emotional resistance to doing what his father told him. He needed no one else's permission to do it. He could carry on the Jewish mission with a perfect purity. But he did need a miracle. After leaving his father's house, he experiences a dream on Har HaMoriah, where Avraham sacrificed Yitzchak and where his own children would build the Beit Hamikdash. This bridge between the past and the potential future, the covenants of heaven and earth, formed the ladder. Here G-d repeated the covenant that his children would become a multitude and that he would be brought back to the land when all that G-d had spoken of would be fulfilled.
When Yaakov asked his son to swear, he was making him into a partner in carrying out G-d's promise. Just as Avraham had made Eliezer into a partner in carrying it out. By bringing Yaakov back to Avraham and Yitzchak, he was marking a place on a physical and spiritual map, not only an individual resting place, but a national rallying point. A plan that connected the future of his children at the end of days, with the first work of his fathers. All part of the greater covenant into which the oaths flowed. The oaths which stated that the life of the land and the people were one. That the Jewish people could not exist without a land. And the land of Israel could not exist without its people.
They might be separated at times through exile, but they would always return. Out of exile, even in death, they would return.