The Collaborative International Dictionary of English v.0.48:

Vermin \Ver"min\, n. sing. & pl.; used chiefly as plural. [OE.
   vermine, F. vermine, from L. vermis a worm; cf. LL. vermen a
   worm, L. verminosus full of worms. See Vermicular, Worm.]
   1. An animal, in general. [Obs.]
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            Wherein were all manner of fourfooted beasts of the
            earth, and vermin, and worms, and fowls. --Acts x.
                                                  12. (Geneva
                                                  Bible).
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            This crocodile is a mischievous fourfooted beast, a
            dangerous vermin, used to both elements. --Holland.
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   2. A noxious or mischievous animal; especially, noxious
      little animals or insects, collectively, as squirrels,
      rats, mice, worms, flies, lice, bugs, etc. "Cruel hounds
      or some foul vermin." --Chaucer.
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            Great injuries these vermin, mice and rats, do in
            the field.                            --Mortimer.
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            They disdain such vermin when the mighty boar of the
            forest . . . is before them.          --Burke.
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   3. Hence, in contempt, noxious human beings.
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            You are my prisoners, base vermin.    --Hudibras.
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WordNet (r) 3.0 (2006):

vermin
    n 1: an irritating or obnoxious person [syn: vermin,
         varmint]
    2: any of various small animals or insects that are pests; e.g.
       cockroaches or rats; "cereals must be protected from mice and
       other vermin"; "he examined the child's head for vermin";
       "boys in the village have probably been shooting vermin"

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "vermin":
   amphibian, animal, aquatic, beast, biped, canine, cannibal,
   carnivore, cattle, chaff, cosmopolite, cur, dog, dregs,
   dregs of society, feline, gnawer, herbivore, hound, hyena, insect,
   insectivore, invertebrate, mammal, mammalian, marsupial,
   marsupialian, mongrel, offscourings, offscum, omnivore, pig,
   polecat, primate, quadruped, raff, reptile, riffraff, rodent,
   rubbish, ruminant, scavenger, scum, serpent, skunk, snake, sordes,
   swine, swinish multitude, trash, ungulate, varmint, vertebrate,
   viper, whelp, worm