Joe Vitt is going nowhere. He is the Assistant Head Coach for a reason and he took over as interim head coach for a reason. Payton trusts him more than anyone else on the staff. In fact, if Payton can not find the "right fit" as a new defensive coordinator or if the right fit decides to go somewhere else or stay where they are, I could even see Vitt being named Defensive Coordinator.
The guy simply brings the attitude and energy to the team that Payton wants.
As far as LB play, it's not like he has been given a huge amount of talent to work with and he actually did a good job developing Jo-Lon Dunbar and Cassilas from undrafted free agents to at least sometime starters. He even knew how to get the best out of Shanle despite his athletic limitations.
truth is many teams in a 3-4 have two, one for inside and one for olb/pass rushers