The 1981 edition, along with the two 20th-century RLDS editions, has the original thy. The change to the first showed up in the 1840 edition, probably as a typo. The incorrect the entered the LDS text in the second printing of the 1852 British edition; for that printing, the stereotyped plates were corrected from thy to the by consulting the 1840 edition. Throughout the history of the text, the visually similar determiners thy and the have often been mixed up. (For a list of instances in the manuscripts and the 1830 edition where thy was replaced by the, if only momentarily, see the discussion under 2 Nephi 20:30.)
Elsewhere in the text, there are no examples of “the land of the vineyard”, but in the olive tree allegory there are two examples of “the land of my vineyard” (in Jacob 5:43 and Jacob 5:69). So we do expect a possessive pronoun like my or thy before vineyard in the allegory.
Summary: Retain in Jacob 2:21 the original possessive pronoun thy in “the land of thy vineyard”.