, there's another forum member that's running your vintage Vigilant w/coal. rewinder
is his name. He recommends adding a piece of gasket material at the bottom of the door so the gap between the door and the fettle is sealed. Doing so prevents any combustion air from bypassing the fire bed. You might have seen the picture earlier in this post
on this very thread. Search around this forum for his posts
or he just might chime in soon
The problem you describe was one that I heard many times from owners both now and in the era when the combo unit was introduced. I've never owned nor operated one with the insert, but I recall sealing all pieces together to prevent the air from bypassing the fire bed was necessary to get a successful burn. That was especially so when the internal damper was closed as the chimney's draft was reduced to hold the heat in the stove longer for better heat transfer.
Here are a links to a thread you might find helpful:
I just purchased a gasket set for my Vigilant II and replaced the door gaskets. The kit came with gasket material for the doors, windows, top griddle, enough door gasket left over to possibly (?) do the ash drop on my model, and finally a tube of cement. Your model doesn't have the ash drop. The doors took a 5/16" gasket. IIRC, it was ~ $25.