In 2 Nephi 3:9, we read, "He shall be great like unto Moses." Doctrine and Covenants 28:2 states that "no one shall be appointed to receive commandments and revelations in this church excepting my servant Joseph Smith, Jun., for he receiveth them even as Moses." Then in D&C 107:91 we read, "And again, the duty of the President of the office of the High Priesthood is to preside over the whole church, and to be like unto Moses." By fulfilling these priesthood responsibilities, Joseph Smith fulfilled this prophecy of Lehi.
In 2 Nephi 3:11 it is prophesied that this seer will "bring forth my word unto the seed of thy loins … to the convincing them of my word, which shall have already gone forth among them." The word that had already gone forth is the Bible. Joseph Smith brought forth the Book of Mormon, and the Book of Mormon was written to the convincing of Jew and Gentile that Jesus is the Christ, proving to the world that the holy scriptures, the Bible, that the holy scriptures are true. The Book of Mormon proves that the Bible is true. It is not the other way around. And that also is the fulfillment of the prophecy of Lehi.