"And the time drew near...and he called his son Joseph"
Rabbi Chizkiyah begins by explaining that Israel saw the torture of exile upon his descendants, and consequently, his soul began to die. Israel then gathered his exiles and told them that if they wished to be absolved of their sins, they must behave according to truth and justice in the eyes of God. Rabbi Shimon then explains why Israel called only Joseph his son and not all the other exiles. This is because all the children of Yisrael were called Joseph. Rabbi Chizkiyah then refers to the secret of the Vav as including Jacob, Joseph, and others, as well as the six Sfirot.
Israel's action of talking to the people of the exile is a code for the great Patriarch calling upon all generations, including the readers of this passage, and awakening them to the power of the Light and the path to eternal freedom, fulfillment, and the final redemption. When Yisrael calls only the name Joseph, the Zohar is indicating a direct reference to the Sfirah/dimension of Yesod, the portal and funnel through which the supernal Light flows into our world. The name Joseph refers to the Light that is flowing from Yesod to us - the reader and all the people who remain in exile in our current generation. This Light now becomes part of our being.