The OT from the KJV uses the hebrew and aramaic language not the vulgate.
All the texts of the Septuagint base from the "Letter of Aristeas" where the supposed librarian of the Greek Pharaoh, Ptomely 2 Philadelphus asked the high priest for the Hebrew Bible ( Old Testament) to be translated into Greek for the Alexandrian Jews.
Along with, 72 Jewish scholars put in separate cells where they "miraculously" translated word for word the same. So they claim this Septuagint texts existed in the time of Christ & that he used that instead of the preserved Masoretic texts.
Aristeas claims to be Greek court official sent by the "librarian" Demetrius to gather the Hebrew scholars & naming them but they do not match as hebrew names but rather Greek names in the Maccabean era. Demetrius, the supposed librarian in fact, was never librarian of the pharaoh, only served in his court.
In Aristeas 7:14, Ptomely the pharaoh tells Demetrius & the jewish scholars how wonderful it is that they came on fhe anniversary of his naval victory over Antigonus". When the ONLY recorded Egyptian naval victory happened many years after Demetrius death, proving it to be a hoax.
Many Christian scholars claim that Jesus used the septuagint in his time but that contradicts scriptures (see below) where a jot is a hebrew letter & a tittle being a small mark to distingius between hebrew letters. Also, Jesus only mentioned the scriptures in the traditional hebrew way Torah (Law), Nevi'im (prophets), Ketuvim (writings).
Matthew 5:18 - For verily I say unto you, Till heaven and earth pass, one jot or one tittle shall in no wise pass from the law, till all be fulfilled.
Luke 24:44 - And he said unto them, These are the words which I spake unto you, while I was yet with you, that all things must be fulfilled, which were written in the law of Moses, and in the prophets, and in the psalms, concerning me.
The reason these texts are still being brought up is of what I said earlier the Roman Catholics & Othodox desperatly want them to be genuinely inspired bc it goes with their doctrine. 45 Alexandrian manuscripts vs the 5,000 Greek manuscripts favoring the textus receptus. The septuagint texts are the ones that started to canonize the apocryphas forcing you to accept everything that the doctrine contains which are leading many protestants to Rome.
"…the Septuagint, the Greek translation from the original Hebrew, and which contained all the writings now found in the Douay version, as it is called, was the version used by the Saviour and his Apostles and by the Church from her infancy, and translated into Latin, known under the title of Latin Vulgate, and ever recognized as the true version of the written word of God" —Preface,1914 edition.