Where does monstrous vermin or bug appear in the text?

The following description of Gregor can be found in the very first line of the text. The words Monstrous, vermin, and bug are not included in the text.... the word insect is used instead.

AS GREGOR SAMSA awoke one morning from uneasy dreams he found himself transformed in his bed into a gigantic insect. He was lying on his hard, as it were armor-plated, back and when he lifted his head a little he could see his dome-like brown belly divided into stiff arched segments on top of which the bed quilt could hardly keep in position and was about to slide off completely. His numerous legs, which were pitifully thin compared to the rest of his bulk, waved helplessly before his eyes.

English translation may describe him as a gigantic insect, but in the original German it's "ungeheures Ungeziefer."

Opening sentence of the story is,

Als Gregor Samsa eines Morgens aus unruhigen Träumen erwachte, fand er sich in seinem Bett zu einem ungeheueren Ungeziefer verwandelt” = "When Gregor Samsa one morning awoke from un-peaceful dreams, he found himself in his bed transformed into an "ungeheures Ungeziefer".

The phrase has no one-to-one translation into English. Closest translation for "ungeheures" = "enormous" but if you think about that word, it means not just "very large" but also "outside the norm" or overtones of "abnormal."

"geheuer" is only used in the negative ("nicht geheuer") and means something like "suspect, fishy, dubious, something's wrong with that thing", so "ungeheuer" again suggests "something's not right or not natural with that thing" so you can wind up somewhere near "monstrous," so I could see how a translator could consider that word.

"Ungeziefer" = "vermin" but is suggestive of of "unclean animal, unfit to be offered to God as a sacrifice." Maybe think "tainted, nasty, polluted, disgusting, etc." "Insect" is entirely indefensible based on the German of this sentence, although the creature described later has many insect-like traits in terms of how its legs apparently work, how its body is structured etc..

So you can see how being specific about the phrase meaning "large-in-size" "insect/bug" gets it wrong. You might be able to get away with "enormous vermin" without betraying the original meaning too much.

https://oomkenscom.com/2013/06/26/kafkas-ungeziefer-in-the-metamorphosis/ is a good read on some of the nuances around the term. It also points out that over the course of the story Gregor's description and his animal attributes don't really align with an insect and even ultimately contradict each other.