"Detestable Vermin" was an umbrella term for organizations opposed to the reanimated and infested Alexei Stukov. The term comes from an insult Stukov directed at them, rather than their own name.
The "Vermin" consisted of the Kimeran Pirates, a group of protoss fighting alongside mercenary archon Mertick the Mauler and a group of zerg fighting alongside "mercenary" mutalisk Cardis the Destroyer.
The groups took shelter on a dark planet, but Stukov tracked them. He hired a selection of mercenaries to fight and destroy them. The "Detestable Vermin" were destroyed.
2001-01-31. StarCraft: Brood War. Vivendi Games. Mission: Mercenaries II (in English). Map Archives: Mercenaries II.