An insect doesn't have quite the same meaning as a vermin. An insect is a biological term, and it can have a variety of connotations: there are beautiful insects (such as butterflies or ladybugs), they may be big or small, associated with different human sentiments, some of them are even edible, etc. So, this term is somehow devoid of a specific meaning.
On the other hand, a vermin is quite an unambiguous word. From a human viewpoint, it is an animal (not necessarily an insect!) which employs a destructive behavior, and it is very repulsive. Vermins are cockroaches, flees, lice, ticks, even rats. So, it has a distinctly negative connotation. Kafka's Gregor Samsa, after his transformation, is forced to hide in his room, out of his family's sight, as he is an abomination now. His verminous nature also suggests that he is a burden to his family, that he doesn't produce anything, doesn't provide for his family. He devours garbage and leftovers, which makes him even more disgusting, even to his mother.
