User blog comment:Zippertrain85/Vampires are NOT undead in TES/@comment-25225642-20140727192845

I think the vampires being something between makes more sense, since I don't think Bethesda would classify them as undead only to make a spell more useful. About the rest, you have the point.

PS. Who uses Detect Life/Dead in Skyrim when you have Aura's Whisper :D.