The damper to the vertical chamber allows me to get the VC the same temp as the HC. It also allows me to dump heat/smoke to the HC a little bit As you have noticed with your VC the temp is much more stable to spikes and such. I have used it for both warming and cooking. IE..If I want to do a prime rib and I want a very precise 225 deg, I use the vertical chamber because I can control the heat very precisely (note I'm not doing volume cooking like you). What I do love about it is the ability to do vertical fire cooking....ie the coal pan and fire or coals. This is what inspired the XFire Grill build. To me this adds the most functionality to the VC as anything. I would NOT have a VC without this function!!! Given what you said about wood and such, I would lean toward insulating the VC as that will make it even more of a cooking chamber (This will add lots of volume for your large cooks). It may raise the temp for warming and such though, but you can always open the vent (if you add one for fire cooking) to cool it down. I also think the insulation will off set the smaller fire box the Lang has vs the chamber volume. Since you have the VC insulated the FB size should be less of an issue.
Here and example of the VC under an active fire. I have some LTales on the on the top shelf and open flame searing some NYs. This fire with the door closed with make the VC about 375-400 deg. I have also added spuds, beans, and other sides in the VC to cook while the HC is smoking, I would suggest a bigger ash pan than what I have and add a lower coal grate to sit above to allow for air flow and better heat control. This coal grate shelf could also serve as a place for a deflector plate/water pan to smoke vertically. If it is a plate make sure it is smaller than the ash pan so any grease will be captured.
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On to the gas burner...….I have spent lots of time thinking about this and the #1 concern is gas build up in a confined space. The fire box of a smoker is a very dirty place and that doesn't play nice with sensitive gas equipment, ie.. thermocouple's, pilots, burners, etc. Having a mishap is simply NOT an option!!!! So I have thought about adding a pipe burner in this fashion (IE similar to the common BBQ design).....Sorry but I have to note, this is an un-tested design and when working with gas, safety should be the #1 consideration!!! Making a mistake with gas has and does take life so that should be the #1 driver in any design.
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The pipe burner (I happen to have a nice one from a Capital Gas Range, was saving for a built in bbq build) would be attached to the side of the FB in an isolated box with some type of gate on the top. This gate could be as easy as a sliding removable plate on the inside of the FB vs a swinging one. It just needs to be in place when you want to do fire heat. The gas burner box (GBB) would be open below to allow an gas to evacuate in the event a blow out occurs and the fail safe vales fail. My thought is a pilot and thermocouple similar to a outdoor fire place would be a secondary safety feature incase of a blow out (un like a common BBQ that have even gone boom when a blow our has occurred and the lid wasn't open to let gas dissipate prior to re-light). Next a higher removable burner shelf would be added so wood chucks splits could be added to add the flavor smoke if desired. This could also just be a large pellet log or maze to keep it simple and efficient....
That said......The pellet log and pipe burner would be a very consistent and set it and don't touch it but NOT forget it......this is just me but I will not leave a grill of any kind un-attended.....even a pellet grill......I learned this the hard way with a pizza night after the PID on my Memphis glitzed and it went into grill mode and actually lit a couple of nice racks of beef ribs on fire. Yes when I opened the grill lid the bones were burning like wood. I wasted 3 lbs of salt putting the flames out not to mention we had to eat pizza and not ribs!
The final thought on the gas burner application is that of efficiency, a RF smoker is not very efficient so it could burn a lots of gas to keep it at a proper temp........your 84 has a lot of volume!