I haven't seen anything canon regarding Vampire Spawn souls' afterlife, and couldn't find anything on them going to Mephis. I don't think they would unless they got turned into soul coins or something. My best guess, since the deities hate undead and probably won't claim them, is that they get added to the Wall of the Faithless, but I think that Wall was retconned, so they probably just walk the Fugue Plane eternally. Maybe go to the City of the Dead. Not a nice afterlife, but Karlach (if she dies) gets the same one according to Withers. Faerunian Afterlives suck really bad and I hate how the lore works here.
