I have it in my copy of the Vulgate, but it is not in the Jesuit Douey-Rheims.
You are correct. I checked my 1950 printing of the 1826 Douay-Rheims (one of my prize possessions -- I use it to taunt Catholics on how they used to use pretty good English in their translations, unlike the schlock they put out now), and it is not there. But it is in the Clementine Vulgate. Interesting.
The Johannine comma is put into parentheses and a footnote is added, basically saying that few MSS have this phrase, but the editors quickly note that if the Pope says it belongs, it belongs!