I think the point, although Rourke isn’t entirely clear and my Prague German dialect from a hundred years ago is a bit rusty, is that ungeziefer, while literally meaning “vermin,” was used to refer specifically to cockroaches. So Kafka wasn’t being ambiguous about the type of insect Gregor turns into. Personally, I prefer vermin too, but that’s based on my familiarity with the traditional opening and my preferred reading of the text, not on what Kafka actually wrote.
When it comes to literary translations, you really should consider them to be works that stand on their own. It is impossible to convey the exact subtleties, tone, register, prosody, and style of the original when translating, at least not in the exact same places as the original. Wordplay and polysemy are almost always lost. A translator can substitute, using wordplay in a different passage for example, to mimic the style of the original, but the result is a new work, not a version of the original.