Source: Apolog. resp. inuect. Ed. Lei(Apologia qua respondet duabis inuectiuis Eduardi Lei), ASD IX-4, pp.54-55 ll. 894-914. Translation Erika Rummel in CWE 72, p. 44.
So much for the supposed admission of back translation in his Annotations. The Greek for the last few verses (or just v 19 depending upon whom you read) was provided from the Greek manuscripts of the Aldine printers. The idea that Erasmus would emend his Greek text from the Latin Vulgate was contrary to the very thing he was producing: a fresh Latin translation! It is the height of absurdity to claim that he back translated from the Vulgate into Greek and then retranslated his Latinized Greek back into Latin.
And let us not forget the episode with Erasmus and the Comma Johanneum. It was not printed in his first 2 editions. Now this should strike his critics who accuse him of back translation as very odd indeed. Why didn’t he simply back translate the CJ from the Vulgate and provide the Greek text for it that and thereby avoid the controversy? All that time and effort spent on procuring a Greek text with the verse could have been avoided as well. And yet, he chose not to print it because he had no Greek manuscript which contained the verse. That alone tells us all we need about the last 5 verses in Erasmus Greek New Testament. From Erasmus’ own behavior in dealing with the CJ, we can conclude that he would not put a back translation of the Vulgate in place of the Greek text.