The section of Vayelech (and the next two Parshiot of Haazinu and V’Zot HaBerachah) describes the events and words spoken on the last day of Moses’ life:
Moses went and spoke these words to all Israel. He said to them: “I am a hundred and twenty years old this day; I can no longer go out and come in, and G‑d has said to me: You shall not cross this Jordan . . .”
Moses entrusts the leadership of Israel to Joshua. He puts the Torah into writing, and commands them the mitzvah of Hakhel (“gathering”): every seven years, on the Sukkot festival following the Shemittah year, “Gather the people together, men, women and babies, and your stranger that is within your gates”; the king shall then read from the Torah to them, “that they may hear and that they may learn, and fear the L‑rd your G‑d, and observe to do all the words of this Torah.”
Moses again warns of the hiding of the divine face which shall occur when the people abandon the Torah; indeed, G‑d Himself predicts that this will be the case. “This song” must therefore serve as an everlasting testimony to ensure Israel’s eventual return and rapprochement with their G‑d.