Shadow Vermin is one of the monsters you have to kill in order to get the Beastmaster of The High Heavens Achievement in Diablo III. After you have killed a Shadow Vermin you automatically get its Beastiary Lore Book. Listed below are the areas in which you might encounter the Shadow Vermin
Shadow Vermin - Gardens of Hope 2nd Tier
More guides can be seen at the Diablo III Forums. The complete list of all monsters can be found at the Diablo III Beastmaster of the High Heavens Achievement Guide. If you have more info about the Shadow Vermin please post them below.