1) Jerome begins by noting that the Septuagint is not a pure translation:
If the version of the Seventy translators is pure and has remained as it was rendered by them into Greek ... Now, in fact, when different versions are held by a variety of regions, and this genuine and ancient translation is corrupted and violated, you have considered our opinion, either to judge which of the many is the true one, or to put together new work with old work, and shutting off to the Jews, as it is said, "a horn to pierce the eyes."- Jerome, Prologue to Chronicles
2) Jerome continues by noting that in his day it was famous that there were three regional varieties of the Septuagint:
The region of Alexandria and Egypt praises in their Seventy the authority of Hesychius; the region from Constantinople to Antioch approves the version of Lucian the Martyr; in the middle, between these provinces, the people of Palestine read the books which, having been labored over by Origen, Eusebius and Pamphilius published.- Jerome, Prologue to Chronicles
3) Jerome argues that although Jesus knew the Septuagint translation, he used the Hebrew, arguing from various passages:
I have recently written a book, "On the best kind of translating," showing these things in the Gospel, and others similar to these, to be found in the books of the Hebrews: "Out of Egypt I called my son," and "For he will be called a Nazarene," and "They will look on him whom they have pierced," and that of the Apostle, "Things which eye has not seen, nor ear heard, and had not arisen in the heart of man, which God has prepared for those loving Him." The Apostles and Evangelists were certainly acquainted with (the version of) the Seventy interpreters, but from where (were) they (supposed) to say these things which are not in the Seventy?- Jerome, Prologue to Chronicles
4) Jerome notes that the church of his day did not accept the apocrypha, but only the Hebrew books, as can be seen from the middle of Jerome's punchline for his argument about the Septuagint:
Certainly, whatever is witnessed by the Savior to be written, is written. Where is it written? The Seventy don't have it; the Church ignores the apocrypha; thus the turning back to the Hebrew (books), from which the Lord spoke and and the disciples took forth texts.- Jerome, Prologue to Chronicles
5) In the conclusion of the prologue, Jerome explains the fact that he was coming under a lot of fire for his new translation, since popular opinion was fond of (their own version of) the Septuagint:
In peace I will say these things of the ancients, and I respond only to my detractors, who bite me with dogs' teeth, slandering me in public, speaking at corners, the same (being) both accusers and defenders, when approving for others what they reprove me for, as though virtue and error were not in conflict, but change with the author. I have recalled another edition of the Seventy translators corrected from the Greek to have been distributed by us, and me not to need to be considered their enemy, which things I always explain in the gatherings of the brothers.- Jerome, Prologue to Chronicles
Thanks very much to Kevin P. Edgecomb who provided this translation and released it into the public domain.
-TurretinFan
UPDATE:
One Roman Catholic reader (I'm not sure whether he'd want attribution or not, so I've not given it to him for now. If he wants it, he knows how to let me know) pointed me to the fact that one can find translations of many of the prologues to the Vulgate books (link). Some have suggested that the later prologues show Jerome softening in his opposition to the apocrypha, though you will note:
Also included is the book of the model of virtue Jesus son of Sirach, and another falsely ascribed work which is titled Wisdom of Solomon. The former of these I have also found in Hebrew, titled not Ecclesiasticus as among the Latins, but Parables, to which were joined Ecclesiastes and Song of Songs, as though it made of equal worth the likeness not only of the number of the books of Solomon, but also the kind of subjects. The second was never among the Hebrews, the very style of which 18is redolent of Greek speech. And several of the ancient scribes affirm this one is of Philo Judaeus. Therefore, just as the Church also reads the books of Judith, Tobias, and the Maccabees, but does not receive them among the canonical Scriptures, so also one may read these two scrolls for the strengthening of the people, (but) not for confirming the authority of ecclesiastical dogmas.- Jerome, Prologue to the books of Solomon
Similarly:
This prologue to the Scriptures may be appropriate as a helmeted introduction to all the books which we turn from Hebrew into Latin, so we may be able to know whatever is outside of these is to be set apart among the apocrypha. Therefore, Wisdom, which is commonly ascribed to Solomon, and the book of Jesus son of Sirach, and Judith and Tobias, and The Shepherd are not in the canon. I have found the First Book of the Maccabees is Hebrew, the Second is Greek, which may also be proven by their styles.- Jerome, Prologue to the Book of Kings
Yet it was demanded of Jerome that he translate the Apocrypha, to which command he grudgingly complied:
I do not cease to wonder at the constancy of your demanding. For you demand that I bring a book written in the Chaldean language into Latin writing, indeed the book of Tobias, which the Hebrews exclude from the catalogue of Divine Scriptures, being mindful of those things which they have titled Hagiographa. I have done enough for your desire, yet not by my study. For the studies of the Hebrews rebuke us and find fault with us, to translate this for the ears of Latins contrary to their canon. But it is better to be judging the opinion of the Pharisees to displease and to be subject to the commands of bishops. I have persisted as I have been able, and because the language of the Chaldeans is close to Hebrew speech, finding a speaker very skilled in both languages, I took to the work of one day, and whatever he expressed to me in Hebrew words, this, with a summoned scribe, I have set forth in Latin words.- Jerome, Prologue to Tobias
Likewise:
Among the Hebrews the Book of Judith is found among the Hagiographa, the authority of which toward confirming those which have come into contention is judged less appropriate. Yet having been written in Chaldean words, it is counted among the histories. But because this book is found by the Nicene Council to have been counted among the number of the Sacred Scriptures, I have acquiesced to your request, indeed a demand, and works having been set aside from which I was forcibly curtailed, I have given to this (book) one short night’s work translating more sense from sense than word from word. I have removed the extremely faulty variety of the many books; only those which I was able to find in the Chaldean words with understanding intact did I express in Latin ones.- Jerome, Prologue to Judith (It's not clear to me whether Jerome was being confused or sarcastic. Nicaea did not decide the canon, and had they done so, one would hardly expect the later councils of Hippo and Carthage to omit reference to this fact.)