Dear senior member Jeepboy,
I've BTDT on the 123. If the center vent isn't opening, verify with a vac source, and prepare to remove the entire upper dash. You can get one or two fingers on it with the upper dash installed, but I've yet to hear of anyone who ever pulled it without removing the dash. That includes me.
The first (and hopefully last) time you pull the upper dash, plan on 4 - 5 hours for the r/r. Have replacement diaphragms on hand or be prepared to leave the car sit torn apart. I don't prefer this method because my memory for how everything came apart fades exponentially with each passing day. After three or four days, I'm relearning what I did on the disassembly.
Once you have the upper dash out, check all of your vacuum actuators (that's what I call them) and replace or repair any that are marginal. Some would say r/r any that are hard to get to at all, your choice. Also check the vacuum solenoids behind the switches in the center console. They aren't easy to get to, but they're EASIER with the upper dash out. They're nickel/dime parts that kill the system when they fail so check them also.
Before you put your dash back in, check all of your vacuum lines and connectors for cracks and disconnects caused during the job so far. When the dash is back in, check again. Also, there is a check valve in the very front of the dash behind the speedo assembly, all the way up under the dash surface when it's installed. Check that one too, it's a royal PITA to change with the dash intalled.
That's all I remember, I did mine over a year and a half ago. If you get stumped on the removal, you probably are missing at least one fastener. The dash doesn't need a lot of force to come out, just careful maneuvering around the steering wheel.