I assume you're talking about the actual vent window frame. You'll definitely want a copy of the obscenity encyclopedia before you start. It'll take every single word for you to get throught the job. I also recommend suicide prevention counseling.
You'll have to take the entire vent window frame out, which means you'll have to remove enough felt from the window itself to get it out. This is obviously the best time to replace all of it--that part is easy. You'll have to tilt the top of the vent aft to remove it. There are screws in the door under the vent latch, on the upper vent frame, and on the front of it (four total, I think). None of the top 3 will want to go back in when you're done. I also had one shear off in the top...oh joy.
I pried the upper part of the frame (short side) away from the glass with a screwdriver, then worked my way around. It wasn't easy, and was the hardest part of the job. You'll also have to deal with ruining the black seal that goes between the glass and frame. I didn't find anything that even came close, so I ended up making do. I also replaced all of the other vent seals. Let me know if you have anymore questions or can further brighten your day.