Yes Matt, your statements about the secondary air inlet on the left side of the stove are correct imho. If your fuel of choice is anthracite, the port doesn't have much function other than bleeding off some chimney draft, bypassing the firebox just like any other leak in the stove will do above the grates. On the other hand if your choice of fuel is bituminous, the port might
have some secondary combustion function. For anthracite, the secondary air is thru the unsealed bottom of the door glass. Secondary air for an anthracite fire needs to be close to the top of the coal bed - and very little of it too.
The Vigilant's exterior castings have been around since the model was released. The secondary air inlet followed them with the unchanged stove side casting. Modifications made to this model by VC began within the stove. If you chose the multi-fuel version, a multi-part hopper/grate system was needed to switch between using wood or anthracite as a fuel. I understand that VC made a coal version of this older model but the main difference was the grade of iron used in the casting. As the model changes moved the Vigilant toward a coal "only" stove, VC added glass to the larger front doors,grate design changes, more room for the larger ashpan and, eventually, the swing out ashdrop assembly, the stove-within-a-stove design and the integrated internal by-pass damper used on the model Vigilant II 2310. The exterior of the Vigilant variants staid essentially the same.
Going back to the port, it was originally designed to add pre-heated secondary combustion air into the side chamber to help support secondary combustion of wood smoke. This was old tech, not the new air systems and cats we see in modern EPA wood burners. My 70's vintage VC Defiant had the exact same cover over its secondary air inlet. It did serve a function, maybe not so much in introducing secondary air to support secondary combustion of the wood smoke, but more so to prevent puff-backs when the wood fire was barely burning and the smoke would linger in the stove. When the thermostatically controlled main air would close, the wood smoke would barely move away from the fire. As little O2 made it into the fire box, the smoke would continue to linger near the hottest part of the fire and reach it's ignition ratio of fuel/air/temperature until it would ignite in minor explosion - puffbacks not unlike we occasionally see with coal but much smaller. Small ammounts of wood smoke would puff out of the main air intake and then the cover would slam shut as the mini-explosion caused the column of smoke in the stove to expand a little, then as it settled it would again mix with O2 until the conditions were again right for another PUFF. Once the rhythm was established, there would be a chain of continuous puffs and you would hear the thermostat cover banging closed after each PUFF. The first time I encountered this, it sounded like a toy locomotive running on its track: chink-chink-chink-chink-chink ... . The room got smokey enough to set off the smoke alarm and wake me up in the middle of the night! Anyway, telling that story got a little long, sorry. To solve this, you had to close down the secondary air inlet a little to prevent O2 from being sucked back in to the combustion chamber and prevent the puffback cycle from occurring.
All said and done, IMHO, the air inlet on the side of our coal stove is vestigial. It use to have a purpose in a former design employing old technology that became outdated by better designs to accomplish the same function more efficiently to meet new EPA regs. Other VC wood stove models exterior appearances changed along with the modifications required to meet the newer EPA regs. The Vigilant's exterior staid much the same as it morphed into the "coal only" stove, the model II 2310.
I'm going to bed!