the artist's really should explain these things to us but I think she wants hope dead so she can have Light all to herself and Fang is the hopeless bystander...
Vanille is trying to cast 'Death' on Hope to leave the pathway to Light open only for her since no one else in the group (besides Sazh or Fang, very little possibility) has any interest in Light...
Or Vanille is interested in Hope? There is that short scene where she gets flusttered because Hope claims "Her smiling face makes him happy"...Or she could be pissed because he got her good.