I have chosen Aberration, myself.
Vermin Host, Cockroach: Aberration; 5 ft./5 ft.; Hide +5, Listen +6, Move Silently 5, Spot +6, Survival +5; Toughness, Track; LA +3; Saves become Fort +5, Ref +2, Will +5; Split allows the Vermin Host to take the form and abilities of a xxx swarm; Gains Shapechanger subtype.
Vermin Host, Leech: Aberration; 5 ft./5 ft.; Hide +6, Listen +3, Move Silently +6, Spot +3, Swim +10; Stealthy, Toughness, LA +3; Saves become Fort +3, Ref +2, Will +3; Split allows the Vermin Host to take the form and abilities of a xxx swarm; Gains Shapechanger subtype.
Vermin Host, Rat: Aberration; 5 ft./5 ft.; Climb +9, Escape Artist +2, Hide +6, Listen +4, Move Silently +6, Spot +4, Swim +1; Stealthy; LA +2; Saves become Fort +2, Ref +2, Will +4; Split allows the Vermin Host to take the form and abilities of a xxx swarm; Gains Shapechanger subtype.
Vermin Host, Spider: Aberration; 5 ft./5 ft.; Balance +3, Bluff +3, Diplomacy +3, Forgery +2, Gather Information +3, Hide +3, Listen +4, Move Silently +3, Sense Motive +2, Spot +4; Alertness, Weapon Focus (bite); LA +2; Saves become Fort +2, Ref +4, Will +4; Split allows the Vermin Host to take the form and abilities of a xxx swarm; Gains Shapechanger subtype.
Obviously, my versions are incomplete, as I've noted before. I'd argue that Vermin Hosts would make nice templates, but since I've been too lazy myself...