If the ability goes off, it goes off, even if the animation is interrupted.

The tricky part is that under some circumstances the client will let the animation start to play even when the action is not valid, and thus it will interrupt the animation and no damage is dealt. But weaving oGCDs in is very much a part of BRD glameplay and yes, if you use bloodletter between VB and HS is will go off even if the animation is clipped.