One of the curious features of his prefaces to these books is that he says that they are included by the Jews in a collection of books which they call the Hagiographa. Furthermore--and this is combining information gleaned from both prefaces separately--this collection called the Hagiographa is not a part of the biblical canon and it is held to be of less authority in doctrinal matters.
This is so curious because elsewhere Jerome tells us (accurately) that Hagiographa is the Jewish name for the third section of the biblical canon, following the Law and the Prophets. He says this both in the Prologus Galeatus (i.e., the Preface to Samuel and Kings) and in his Preface to Daniel. On the other hand, in the Prologus Galeatus he classifies Tobit and Judith among the apocrypha.
There is some confusion here about terms. Just what is the Hagiographa? Does Jerome use that term in two different ways? Why would he do that? Does Jerome think that Jews classify Tobit and Judith as Hagiographa or as apocrypha?
To ease these tensions (so it seems to me), an alternative reading was introduced into the text of Jerome's prologue to Tobit and Judith, so that instead of locating them among the Hagiographa, he instead locates them among the apocrypha. This alternative reading actually makes quite a bit more sense when compared with Jerome's other statements on these books. Not only that, but it has also become quite popular in the reception of Jerome's prefaces, because it was the reading printed in the Migne text.
It will not work, however, as there is simply too little manuscript support. The standard critical edition of the Vulgate--the Roman edition produced by the Benedictines (Tobit and Judith published 1950)--lists only a few manuscripts giving this reading for Judith, and none for Tobit. The handy Stuttgart edition does not include the reading apocrypha in the apparatus for either prologue. Thus, it would seem that Hagiographa is the correct reading, and this is amply confirmed by modern editions. (This has not prevented some scholars from continuing to cite apocrypha as the true reading; see here on pp. 28, 43-44.)
As I mentioned, the popularity of the reading apocrypha in these prologues really took hold with its inclusion in the text printed by Migne. At PL 29.23-26, Migne offered a long note--partly taken from Jean Martianay, whose edition of the Vulgate was the first to print apocrypha in these passages--justifying his inclusion of this reading over the reading Hagiographa, which is more widely-attested in the manuscripts. Here I translate this note from the Latin. I have attempted to provide links where I could where more information can be found about the various people or subjects mentioned by Migne. I have not always been able to do this, however; especially if I could not figure out who Migne was talking about (e.g., Driedon, Simonius).
The following note is linked to the word apocrypha in the Preface to Tobit.
So reads the great manuscript codex of the Bible of Cartusian Villanova near Avignon, and so learned men agree that it should be read, such as Leander of St. Martin [a.k.a. John Jones, 1575-1636, Welsh Benedictine monk, from 1599 a member of the monastery San Martín Pinario at Santiago de Compostela, Spain] and many others, by whom it was investigated that the Jews acknowledge no other books as Hagiographa beyond those which constitute the third section of the Hebrew canon. Corrupt editions read with the vast majority of manuscripts Hagiographa, for the Book of Tobit cannot be cut off from the canon or catalog of the divine Scriptures, to use the words of the holy doctor [i.e., Jerome], without at the same time it being cut off from the order of the Hagiographa and put into the apocrypha among the Hebrews. But lest any hateful spite, or contrary man, should resound against us here, let it be remembered that each and every book of the Hagiographa of the Hebrews was mentioned and enumerated by Jerome in his Preface to the Books of Kings, among which in no way he wished to count either Tobit or Judith. He said:
The third order possesses the ἀγιόγραφα, and the first book begins from Job, the second from David...third is Solomon, having three books: Proverbs...Ecclesiastes...Song of Songs. Sixth is Daniel. Seventh Dabreajamin...which book among us is inscribed 1&2 Παραλειπομένων [i.e., Chronicles]. Eighth is Ezra, which also itself both among Greeks and Latins is divided into two books [= Ezra & Nehemiah]. Ninth is Esther. And so altogether the books of the Old Law are twenty-two, i.e., five of Moses, eight of the Prophets, nine of the Hagiographa. But some put Ruth and Cinoth [= Lamentations] among the ἀγιόγραφα, and reckon these books in the calculation of its number, and thus there would be twenty-four books of the ancient Law. ... This prologue of the Scriptures can serve as a helmeted beginning for all the books which we are converted from Hebrew into Latin, so that we may know that whatever is outside these, should be put among the άπόκρυφα. Therefore Wisdom [of Solomon]...and the book of Jesus son of Sirach [= Ecclesiasticus], and Judith, and Tobit, and the Shepherd [of Hermas] are not in the canon.
Thus, Jerome would not at all agree with himself if the Book of Tobit, which in this place he declared should be put among the apocrypha, later in the preface to the same book of Tobit he said that among the Hebrews it was transferred to the Hagiographa. Therefore, lest the holy doctor be believed to have thought things contrary and repugnant to hismelf, the error of the ancient scribes should be corrected toward the fidelity of the manuscript copy praised earlier; for, written down at the first with the greatest diligence, it was also emended with fortunate care, and of better character. But concerning these things again in the Preface to Judith, where for the confirmation of our reading, apocrypha, there is additionally another manuscript codex of the Bible of the Avignon library, collated by the Society of Jesus. [Jean Martianay]
--Many other books both old and more recent read Hagiographa, and there are critics, having learned other things, who prefer this reading especially for this reason: that the book, even if it is not received into the Hebrew canon, being prior, which encompasses books written only in Hebrew, nevertheless it continuously obtains divine authority both among them and among the Fathers of the Church, as is learned from the testimonies of both parts, and from the arguments of Grotius and Sixtus of Siena. Nevertheless, the printed reading, apocrypha, which previously Martianay restored from the manuscripts and defended with good arguments, is especially true, and also is confirmed by both the thing itself and by the context in Jerome both of this passage [i.e., the Preface to Tobit] but also much more clearly in the Prologus Galeatus [= Preface to the Books of Kings], and finally in the Prologue to the Books of Solomon, and in another to Jonah. Also a greater number of manuscript codices stands apart from it, as well as the authority of those who have formerly found written thus in their copies, such as the author of the Glossa Ordinaria, Comestor,Cardinal Hugo, the Marmotrectus, Abulensis, the author of the Preface to the Bible edited together with the Glossa Ordinaria and the work of Lyranus, while the following authorities also support this reading: Driedon, Ambrosius Catharinus, Petrus Garzia Galarza, and recenty Simonius and others. But even from these manuscripts which prefer Hagiographa, either here or below in the prologue of Judith, which passage is the twin to this one, some record at the margin of the book, apocrypha. Thus the interlinear gloss has at this passage, and thus Humphrey Hody attests is contained in a certain Lambeth manuscript at the prologue of Judith, where is noted in the margin in an ancient hand more than 200 years old: "apocrypha is the better reading." The same person says: In another manuscript in the Bodleian (NE. A. 2 1) it is recorded at the prologue of Tobit in an ancient hand, "rather, better--inter apocrypha." Thus apocrypha is read in the prologue of Judith in another Lambeth book of highest quality in two large volumes and in a Bodleian book (NE. F. 3 25) apocrypha is read in both prologues. Among the older writers, whoever believed the reading Hagiographa to be correct established that it was to be taken in a broader sense. So Dionysius the Carthusian and in a third book (Bodleian manuscript F. 107) at the word Hagiographa in the prologue of Tobit it is noted: "that is, apocrypha: certainly Hagiographa is said broadly." And in the prologue of Judith at the same word is placed an interlinear note, "broadly."