I haven't paid attention much re. blade of woe (in fact since we get a sort of instanced animation I wonder if they just remain standing like players do when they are gathering the same survey as you .. you do the gathering but they stand there and stand looking into space :P ), but I have seen a player vampire feeding on an NPC, someone was doing it in the little foyer to the Alinor crafting area the other day .. huge blood funnel from the NPC to them as I ran through it. Thankfully I did not get splashed and stain my outfit.