Cutting out existing steel duct in bathroom vent
One way to cut away the excess metal would be to use a rotary Dremel type tool with abrasive cutoff wheels. This method allows the metal to be removed without bending and deformation that other cutting methods would typically cause. 

If you go this route be prepared to use up a good sized quantity of the cutting wheels. When I am using my rotary tool and have a large amount of cutting to do I have about ten of the mandrels that hold the cutting wheels preloaded with cutting disks. This way when  one wears down or breaks I can quick load another mandrel into the chuck on the rotary tool and continue cutting.