Even Jerome's corrupt Latin Vulgate's oldest copy, Codex Fuldensis (542-546AD), does mention 1 John 5:7's words in Jerome's Preface & states Jerome [who was a Catholic] knew translators were removing 1 John 5:7 out of Greek mansucripts

Latin:
Non idem ordo est apud Graecos, qui integre sapiunt, et fidem rectam sectantur, Epistolarum septem, quae Canonicae nuncupantur, qui in Latinis codicibus invenitur: ut, quia [Mart. quod quia] Petrus primus est in numero apostolorum, primae sint etiam eius Epistolae in ordine caeterarum. (0824A) Sed sicut evangelistas dudum ad veritatis lineam correximus: ita has proprio ordini, Deo nos iuvante, reddidimus. (0825A) Est enim prima earum una Iacobi, Petri duae, Ioannis tres, et Iudae una. (0826A) Quae si, ut ab eis digestae sunt, ita quoque ab interpretibus fideliter in Latinum verterentur eloquium, nec ambiguitatem legentibus facerent, nec sermonum sese varietas impugnaret [Al. varietates impugnarent]:
illo praecipue loco ubi de unitate Trinitatis in prima Ioannis Epistola positum legimus. (0829A) In qua etiam ab infidelibus translatoribus multum erratum esse a fidei veritate comperimus: trium tantum vocabula, hoc est, aquae, sanguinis, et spiritus, in sua editione ponentes; et Patris, Verbique, ac Spiritus testimonium omittentes: in quo maxime et fides Catholica roboratur, et Patris et Filii ac Spiritus sancti una divinitatis substantia comprobatur.
(Hieronymus, Prologus Epistolarum canonicarum; Migne Latina, PL 29.0823-0826; Codex Fuldensis, Ranke, 1868, p. 399).LINK

English: The order of the seven Epistles (meaning the Epistles of Peter, James, John and Jude) in those Greek copyists who think soundly and follow the right faith, is not the same as it is found in the Latin copies. As Peter is first, so his Epistles are placed in the former before the rest. But as I have long since corrected the evangelists according to the rule of truth, so these epistles I have restored to their proper order, which, if arranged agreeably to the original text and faithfully interpreted in Latin diction, would neither cause perplexity to the readers nor would the various readings contradict themselves, especially in that place where we read of the unity of the Trinity laid down in the Epistle of John. In this I found translators [or copyists]
widely deviating from the truth, who set down in their own editions the names only of the three witnesses, that is, the Water, Blood and Spirit, but omit the testimony of the Father, the Word and the Holy Spirit, by which above all [strengthens the Catholic faith (translated)], and places the divinity of the Father, Son and Holy Spirit is proved to be one.
(Prologue to the Catholic Epistles; Translated by 'Ben David' [John Jones], 'Letters to the Editor', The Monthly
Repository of Theology and General Literature, vol 21, 1826, p. 214-215.) LINK
In Jerome's Preface to the Vulgate Version of the New Testament Addressed to Pope Damasus, A.D. 383., Jerome mentioned he used early Greek manuscripts, which proves that 1 John 5:7 was in some early Greek manuscripts that Jerome had & 1 John 5:7 was in Jerome's Latin Vulgate (405AD) as Jerome said:
👉 "in that place where we read of the unity of the Trinity laid down in the Epistle of John. ... translators ... omit the testimony of the Father, the Word and the Holy Spirit"
And👉"You urge me to revise the old Latin version, and, as it were, to sit in judgment on the copies of the Scriptures which are now scattered throughout the whole world;..you would have me decide which of them agree with the Greek original. ...
If, on the other hand, we are to glean the truth from a comparison of many, why not go back to the original Greek and correct the mistakes introduced by inaccurate translators,..I am not discussing the Old Testament,... I am now speaking of the New Testament. .... I therefore promise in this short Preface the four Gospels only, which are to be taken in the following order, Matthew, Mark, Luke, John, as they have been revised by a comparison of the Greek manuscripts. Only early ones have been used."LINK