Oxidation pits vulcanite when it is severe. Bleach can remove or at least soften much of the oxidized material, which exposes the pits. When oxidation is significant, the ONLY way to restore them to deep black, smooth, and shiny is to sand the surface until the pits have been removed, then you can use finer abrasives followed by buffing with rouge to bring it back to like new. You can make them look blacker and dull with deoxidizers and then temporarily shiny with oil, but to do it right, you have to sand.
I recommend starting with a high quality 600 grit automotive wet/dry sandpaper, like 3M. Use it with a few drops of water. Wrap piece of it around a foam nail board to work in corners and to sand the end of the stem near the tenon. This will help prevent rounding over edge, which would be obvious after putting the stem back on the stummel.