Speaking at the LDS Conference in October, 2002, President Gordon B. Hinckley declared that either Joseph Smith's vision in 1820 was authentic or Mormonism is a fraud. The Salt Lake Tribune reported:
LDS faithful believe it all began when 14-year-old Joseph Smith, the church's founder, had a vision of God and Jesus Christ in a grove of trees in 1820.
"Our whole strength rests on the validity of that vision," Hinckley declared. "It either occurred or it did not occur. If it did not, then this work is a fraud. If it did, then it is the most wonderful and important work under the heavens." (Salt Lake Tribune, Oct. 7, 2002, pages A1 & A6)
In 1998 President Hinckley stated that Smith's teaching on the nature of God the Father and Jesus differs from standard Christianity. He also maintained that Smith's 1820 vision gave him a knowledge of God's nature that surpassed that of any minister. Since ministers derive their understanding of God from the Bible, this would demonstrate that Smith's doctrine of God went beyond that source. The Deseret News reported:
In bearing testimony of Jesus Christ, President Hinckley spoke of those outside the Church who say Latter-day Saints "do not believe in the traditional Christ. No, I don't. The traditional Christ of whom they speak is not the Christ of whom I speak. For the Christ of whom I speak has been revealed in this the Dispensation of the Fulness of Times. He, together with His Father, appeared to the boy Joseph Smith in the year 1820, and when Joseph left the grove that day, he knew more of the nature of God than all the learned ministers of the gospel of the ages." (Deseret News, Church News, June 20, 1998, page 7)
Contrary to President Hinckley's affirmation, there are multiple reasons to question Joseph Smith's first vision account.
Smith claimed that from 1820 onward he was persecuted for telling the vision. Yet there is no contemporary evidence that he either told the vision to anyone or even attempted to write it down until 1832. (see Mormonism and the Nature of God, pp. 92-3)
When he did write his 1832 account he only mentioned Jesus appearing. The account never stated that God the Father was present. (see An American Prophet's Record, page 5)
If he had claimed a vision of Jesus it would not have been that different from many Christians of the day who claimed similar experiences. (see Inventing Mormonism, p. 52)
In 1834 the LDS Church magazine printed an account of the beginnings of Mormonism; yet it failed to mention Smith's 1820 vision in the grove. Instead, it related that Smith's first vision happened in 1823 when the angel appeared in his bedroom to tell him of the Book of Mormon plates. (see Messenger and Advocate, Vol.1, no.3, pages 42 and 78)
In 1835 he related the vision in the grove to some visitors, but he only mentioned angels appearing, not Jesus and God the Father. (see An American Prophet's Record, pages 51, 59)
Early newspaper articles criticizing Joseph Smith never raised the issue of a vision in 1820 or that he was teaching that God the Father and Jesus are separate Gods. (see Sunstone Magazine, July/Aug. 1980, p. 27)
Smith's official account of his vision mentions he went into the grove to pray due to an 1820 revival in his neighborhood. However, there is no record of such a revival that year. ( Inventing Mormonism, chapter 2)
It can be demonstrated that Smith's doctrine of God the Father and Jesus having separate bodies developed years after the founding of Mormonism, not because of an 1820 vision. (see Mormonism and the Nature of God, chapters 2 and 3)
The sermons of early LDS leaders show that they thought the first vision was of angels, not God and Jesus. Also, they did not use the first vision to establish their view of God the Father and Jesus as separate entities. (see Journal of Discourses, vol.6, pages 29, 335; vol.2, pages 171, 196-197; vol. 10, page127; vol.12, pages 333-334)
Smith's concept of God the Father having once been a mortal, having a resurrected body, and achieving godhood contradicts the Bible. (see Isaiah 43:10-11; Isaiah 44:6, 8, 24; Jeremiah 23: 23-24)
For a more lengthy treatise on the First Vision story, The Changing World of Mormonism has a full chapter devoted to it, and it can be read online here. I urge you to read it in its entirety.