Somehow, a mutin has fallen between 2 perfectly good panes of glass. This is a vinyl sidelight next to my front door. How hard would this be to fix on my own, and will I lose its energy efficiency?
It's not fixable. You will need to reorder new glass b/c the glass is sealed and taking it apart and reassembling will likely result in moisture being trapped.
Since you need to have the glass unit replaced, or live with it, I would eliminate the mullions.