Lower charge weights mean lower pressures. Every powder needs a certain pressure to burn most efficiently. When the pressure is too low, you get that sooty carbon buildup in your chamber and around the brass. Evidently, the 40 S&W round and chamber build up a higher pressure than the same amount of powder does in a .45. I don't do auto pistols, so I don't know those specifics. But it is universal that when you start dropping powder charges downward, you will eventually start getting that black soot on your casings. VV powders are indeed clean burning, but they are not immune to the laws of chemistry and physics. I use VV N330 for .45 Colt loads and it's the cleanest burning I have found, but I have to run it at a certain pressure.
You will also find that if you are shooting in cold weather, the cases get more sooty than they do in hot weather with the same exact load. It's the same reason, in cold weather the chamber pressure will be lower so the powder doesn't burn as efficiently.
Try increasing your powder weight just slightly until the soot goes away.
