The history that I have read is that Erasmus did not have access to the Vatican library and was therefore unable to view the codex, so he could not have "rejected its readings".
DID ERASMUS HAVE ACCESS TO CODEX VATICANUS?
Simply put, yes. He had access to it through his correspondence with both Bombasius & Juan Ginés de Sepúlveda. Erasmus was supplied with a transcript of 1 John 4:1–3 and 1 John 5:7–11 from Codex Vaticanus by Bombasius. Through Sepulveda, Erasmus was provided with 365 readings of Codex Vaticanus. In Thomas Horne’s Introduction to the Textual Criticism of the New Testament which has been edited by Samuel Tregelles, we find the following on pg xv & xvi...
https://confessionalbibliology.com/2016/05/16/erasmian-myths-codex-vaticanus/
“For the first edition Erasmus had before him ten manuscripts, four of which he found in England, and five at Basle. ... The last codex was lent him by John Reuchlin ... (and) ‘appeared to Erasmus so old that it might have come from the apostolic age.’ He was aware of Vaticanus in the Vatican Library and had a friend by the name of Bombasius research that for him. He, however, rejected the characteristic variants of Vaticanus which distinguishes itself from the Received Text. (These variants are what would become the distinguishing characteristics of the critical text more than 350 years later.)” (Preserved Smith,Erasmus: A Study of His Life, Ideals, and Place in History, 1923). Erasmus was given 365 select readings from Vaticanus. “A correspondent of Erasmus in 1533 sent that scholar a number of selected readings from it [Codex B], as proof [or so says that correspondent] of its superiority to the Received Text” (Frederic Kenyon, Our Bible and the Ancient Manuscripts, 1895; S.P. Tregelles, On the Printed Text of the Greek Testament; cited from Hills).
https://www.wayoflife.org/database/erasmus.htm