This is perhaps the least clear of the verses of the allegory, for it presumes that we are able to understand the referents for the "first" and the last." Although the natural branches in the nethermost regions have been the most recent focus, this verse begins with a command to the servants, and is not related to the previous contexts of the allegory.
In this case, the last and first refer to the time periods of the grafting attempts. The last to occur was the grafting in of the wild branches to the man tree, or the preaching of the gospel to the gentiles. The first would have to be the natural branches sent to the farther reaches, or the "lost" of Israel including (but not limited to) the Lamanites.
The gospel is therefore to come to the world through the gentiles, and the gathering is to bring Israel in to the church which will be a combination of the Jew and the Gentile.
Polemic: From the standpoint of issues of authorship, it is interesting to note that while this allegory clearly invokes the Lamanite/Nephite story from the Book of Mormon, it does not clearly describe the Restoration. One might suspect that a modern author would make sure that the most important aspects of the modern movement would be covered in the analogy, yet this is precisely the point that is missing. We have the result of the Restoration in the missionary work to the world, but there is no part of the allegory that can describe the Restoration through Joseph Smith.
In terms of the allegory, this would be a grafting of the root (pure gospel) onto wild branch (gentile - not genetically related to the covenant) in the nethermost reaches of the vineyard. In the logic of the allegory, this is what would have had to have occurred, but it is missing completely.
While there are very few "proofs" which can be claimed for an ancient authorship of Zenos' allegory of the olive tree (in light of the very similar Pauline allegory) nevertheless we do have a rather interesting negative proof. That is precisely what one would expect of a modern tale is conspicuously absent. At the very point that the allegory could become self-serving for a modern author, that self-serving portion is not to be found, an there is no hint of its necessity from within the allegory itself. The allegory as written, functions completely without any need for a Restoration.
Of course this does not suggest that there was no Restoration, but rather that the perspective an concerns of an ancient author would have produced the emphasis on the gathering, but not been interested in the Restoration that would have preceded it.