Manticore causes every third consecutive basic attack against an enemy hero to deal 5% of the target’s health in damage.
Death Dealer refunds Vault’s cooldown and mana cost if its empowered basic attack kills an enemy within 1.5 seconds of casting Vault. Usually.
However, if that empowered basic attack triggers Manticore’s bonus damage and kills the target, you get no refund.
As far as I can tell, it does not matter which effect kills the target, you get no refund either way.
I don’t know whether this is a bug or a feature, but it really seems like this should not work this way.