Here the 1830 typesetter changed the preposition from in (the reading in the printer’s manuscript) to into. Whenever the verb involves motion from one place to another and the adverb down occurs, the Book of Mormon text nearly always uses the preposition to, unto, or into to describe the place one is going to—that is, the preposition contains the morpheme to. Consider the following sampling from those cases where the preposition is into:
There are only two cases for which the preposition does not contain the morpheme to. And in both cases, the exceptional preposition is in: namely, here in Jacob 5:29 and in 1 Nephi 4:33 (which reads “if he would go down in the wilderness with us” in all the textual sources, including 𝓞). For the second of these, evidence from errors made by scribe 3 of 𝓞 and internal evidence regarding the phrase “in(to) the wilderness” argue that the original text for 1 Nephi 4:33 read “if he would go down into the wilderness with us” (see the discussion under 1 Nephi 4:33).
Here in Jacob 5, all the other instances refer to “going down into (the nithermost parts of) the vineyard”:
The probable source for the error “in the vineyard” in Jacob 5:29 is the use of that very phrase in the following clause:
In anticipation of the following “that we may labor again in the vineyard”, the scribe accidentally wrote down “let us go down in the vineyard” instead of the correct “let us go down into the vineyard”. This error could have occurred as Oliver Cowdery took down Joseph Smith’s dictation or when Oliver copied the text from 𝓞 into 𝓟 (𝓞 is not extant here).
Summary: Accept in Jacob 5:29 the 1830 typesetter’s emendation of in to into; elsewhere the text consistently uses into rather than in in such contexts involving motion and the adverb down; the probable source for the error here is the use of in in the immediately following clause, “that we may labor again in the vineyard”.