I like the flavor, but given that a WCV can already use Incite in combat and can feed from victims thereof, is that really that big of an advantage?
Here's another idea to consider: How about a weapon that is itself an emotion vampire? Give it one or both of the following properties:
1) The weapon is a physical weapon, but it wounds its foe's soul. Although the weapon attacks the physical stress track as normal, consequences inflicted are mental consequences relating to the emotion the dagger feeds from. (Insert requisite joke regarding skewering foes with your lust dagger here.)
2) The weapon feeds on its chosen emotion, inflicting more savage wounds. Attacks made by the weapon can benefit from tags or invokes of aspects reflecting the weapon's chosen emotion.
From a balance perspective, the dagger in and of itself is little more powerful than a normal weapon due to the above powers. A consequence is a consequence, so the fact that a consequence is mental vs physical makes almost no difference. It is a bit stronger against foes with extra physical consequences, and a bit weaker against foes with mental consequences. Consequences are still just as taggable by the wielder as they would be for a normal weapon, since the weapon can benefit from the tags it inflicts. They are potentially less taggable by others, however.
The big payoff for this weapon comes from the interaction between this weapon and Incite. The vampire can now use Incite to inflict aspects via maneuver or consequences, then tag them for a benefit with the weapon.
As a third power (in place of increasing the weapon rating), it might be interesting to allow the wielder's Incite upgrades to affect the weapon, as well. So if the wielder has Ranged Incite, then the weapon can attack a zone away. If the wielder has lasting emotion, the weapon gains +2 to damage.