That is odd. My unit has an oven, a range hood, and a microwave. The microwave is not a combo convection oven/microwave. The vent hood is not part of the microwave, but a separate unit underneath.
I have a factory installed outside vent. Perhaps the oven option includes a vent hood to the outside? ...at least in 2011?
Don't know; just my conjecture.
A couple of things I did last year. The vent is just a cheap piece of (originally) white plastic, and in under 3 years had faded yellow. Even in Michigan, UV from the sun can damage plastics that are not protected. So I painted it white to UV protect it. Looks a lot nicer.
We also get pesky wasps, especially in the August timeframe, and they like to build nests in such things, so I made my own custom vent screen for the range hood.
I am a woodworker, so I am comfortable with doing detail work. I made a block of wood that could slip into the bottom of the vent, that was about 1/16" smaller than the vent opening. I then made a cut-out in another piece of wood so that the first block of wood would fit inside of it, with the same 1/16" gap all around.
This became a two piece "mold", and on my workbench, I sandwiched a piece of aluminum screen between the two pieces - then hammered the "plug" into the cut-out.
I ended up with a piece of screen that looked like this:
The screen slid into the slot of the range vent just perfectly. I used a small amount of sliicone to attach it. Silicone will keep it there, but not so much that it cannot be removed.
In reality, it fit snugly enough that I probably didn't need to use silicone, but I didn't want to lose it.
Here is the final result:
Might be hard to tell but the screen is in the bottom and flush with the vent cover. I put the screen in before painting the vent white.
If you can find a chrome vent cover, that of course would work. But if not, consider painting a white one as chances are those cheap covers are not UV protected.
Before painting. The hood is yellow in under 3 years.
My wife says I over-complicate everything, but I have a perfectionist mentality... so she will just have to put up with my improvements.