Yes I'm curious about this one as well.  I'm wanting to make a miniature puppet of our stage MC to heckle her with via another puppet.  A puppet controlling a puppet!  Sounds like fun!

Try this:
http://swazzle.com/blogs/2006/03/puppet-building-tutorial-ratchet.html

Swazzle posted a tutorial on making a cat. I know it's not a RAT, but close enough to maybe get you started?

By the way, what is a vent type puppet?

By the way, what is a vent type puppet?

I believe it is a hand in mouth puppet designed to be used with ventriloquism . It may come with a bottom and back opening, moving eyes, eyebrows, and what ever the ventriloquist  orders from the puppet builder.

Oh that's what vent means. When I hear vent I think air ducts.

Vent is short for ventriloquist. A traditional ventriloquist doll or dummy (many argue over the use of doll vs dummy) has the opening in the back of the puppet and the heads where originally made of wood with a handle that came down into the body. Now many are made of foam or other products just like a regular hand in mouth puppet or Muppet type puppet is made. 

Yeah my cousin had a dummy. His head was plastic and you worked his mouth by pulling a string. He named it Charlie and it was creepy. lol

Ah so that's what vent means!  Thanks for clearing that up Shawn.  I think I had the idea that the back of the head was opened to put your head in directly, rather than up through the bottom.  I think I saw a website that was building puppets this way and advertising them as vent style puppets.  Either way, ventriloquist dummies (for the most part) and marionettes for some reason kind of creep me out.  I think it's a throwback to Mister Roger's Neighborhood and Lady Elaine Fairchilde from my childhood...  Her emotionless expression and bright red nose and all... However, I'm developing an appreciation for the engineering and creativity it takes to construct them so it's a start on the road to recovery, right?