Tertullian (AD 215, a little over a century before Sinaiticus and Vaticanus)(Adversus Praxes(Against Praxeas c. XXV)) "'And so the connection of the Father, and the Son, and of the Paraclete [lit. Spirit] makes three cohering entities, one cohering from the other, which three are one entity' refers to the unity of their substance, not the oneness of their number."
Cyprian (AD 250)(Treatise I, On the Unity of the Church, sec. 6), "and again it is written of the Father, and of the Son, and of the Holy Spirit, "And these three are one."
Priscillikian (AD 380, within 50 or so years of Vaticanus and Sinaiticus)(Liber Apoligeticus): "As John says 'and there are three which give testimony on earth, the water, the flesh the blood, and these three are in one, and there are three which give testimony in heaven, the Father, the Word, and the Spirit, and these three are one in Christ Jesus.'" (contemporary existence alongside Vaticanus and Sinaiticus. Also note that quotes are added by the later editor, this verse may have ended at "are one" because he's quoting it).
St. Jerome himself (AD 390)(prologue to the Canonical Epistles): "si ab interpretibus fideliter in latinum eloquium verterentur nec ambiguitatem legentibus facerent nec trinitatis unitate in prima joannis epistola positum legimus, in qua etiam, trium tantummodo vocabula hoc est aquae, sanguinis et spiritus in ipsa sua editione ponentes et patris verbique ac aspiritus testimoninum omittentes, in quo maxime et fides catholica roboratur, et patris et filii et spirtus sancti una divinitatis substantia comprobatur.'" This is actually the reason some belive that Jerome's text had 1 John 5:7 in it. I don't have a good translation of this to post, mine's pretty shoddy (though I've read it elswhere). I don't expect you to take my word for it, but in a nutshell, he's saying that it's by the negligence of translators that 1 John 5:7 is not left intact. Also considering this statement, seeing as how it's not found in his Vulgate, it would seem that it conveniently vanished sometime after his death. If he would say irresponisibility caused the omission, why would he then cut it out? The first copy of his Vulgate that we have is the Codex Fuldensis (AD 546). The other major fact is that Jerome never finished his translation, which was finished by subsequent other editors; a lot could have happened in that time.
These are just a few I found,
Information courtesy of Therealbrianw from bibleversiondiscussionboard.com