...8<..I think that cover is a throwback to the wood/coal days.
Yes, I'm pretty sure it is. My Defiant had the same exact secondary air set up. Back in the day before there were cat systems on wood stoves. The concept was to introduce pre heated air into the "secondary burn chambers" to help flash/light off the hot smoke. I thought it might have some use when burning bit that has more volatiles. I tried it with anthracite and just lost draft - worthless for hard coal fuel. I don't know if it ever worked as planned but it did work sometimes. Worthless story alert, for coal burners anyway
About the secondary air hole on the Vigilant....when I burned wood in the Defiant in the spring as it was warming up overnight, the smoke alarm woke me so I hustled down to the stove. Found it chugging away like an old steam piston; chug-chug-chug .... and with each chug a puff of smoke would come out of the secondary and primary holes. Each chug was a mini explosion in the secondary chamber. Not enough draft to pull up the chimney so the O2 starved smoke would settle in the chamber, get enough O2 from the secondary air hole, pop off a mini boom shooting smoke out of the hole. Then it would repeat .... chug-chug-chug... and slowly fill the room with smoke
Had to make sure it was set to a smaller opening for those outside conditions to keep the draft up thru the cooler fire.