Many of us have been using the following artAssets in our skins
<Mapping ArtAssetID="opponent_vitals_backdrop_nemesis" FileName="" />
<Mapping ArtAssetID="opponent_vitals_backdrop_archnemesis" FileName="" />
They are supposed to change the vital panels for Nemesis and Arch-Nemesis class bosses.
I just realized (I might be wrong though) that these two assets are not functional, at least for now. They are not listed in book10 xml template. Would be nice to have them enabled.