I have purposefully done two things in Vermin to (possibly) help you out: left the setting very sketchy, and dropped plot hints all over the place. Here's what comes to mind for me, but I suggest, and hope, you come up with your own unique adventures:

• The Vermin King (or possibly the Vermin Boss for your block) has fallen ill. An old sparrow storyteller swears that it's human poison and that he knows a dusty old building where an antidote could be found--but it's inhabited by an old warlock and his cat--can you make it in and out, and figure out what in the world it is that you need?
• It's war in the streets as the Roaches finally split into two factions. They both want your favor, but the vermin are split among turf and racial lines. How will your (race-mixed) party fall?
• A legendary Toad assassin has gotten possession of twin metal curved swords, rumored to be magical, and is taking down vermin for the highest bidder. Unfortunately, the highest bidder's a giant water moccasin snake, and on top of that, word on the streets is that he was sighted by a human.
• A member of your party was stealing trash outside a window the other night when he heard cries. Climbing up onto a windowsill, he saw a wicked, wicked man beating a smaller man, probably one of his brood. It's human business--but it's wrong. What will you do? How can you stop a huge man, you worthless nothings?

The main thing to remember is that the crap gets awfully high real quick when you're 4 inches tall. Keep your players busy and keep them digging their way out. Have fun, and feel free to send any comments!