Keep in mind the Panthers are using Reddick as a 4-3 off-ball LB which is different than how we were using him. That isn't to say he wouldn't have been successful but he went to a coach who knew exactly how to use him and put him in a position where his size isn't as much of a liability on the edge.
Not shocking that this is the same coach who was able to get immediate contributions from Jeremy Chinn who was a similarly position-less player out of college like Simmons. Some coaches are better teachers than others. Some coaches are able to adjust their scheme to fit players into positions to be more successful than others. Rhule and his DC are obviously better at this than Vance.