Although lords and heroes are vital to the success of every army, it is more the case for the Vampire Counts than any other. Their roster is full of very slow infantry and fast but fragile warbeasts. As with every Undead faction, if a unit's lord is killed or if they are losing very badly, they will start to lose binding and start taking damage over time. They also have hardly any ranged units and no artillery, making them plodding and one-dimensional on the battlefield.
The lords and heroes, however, have access to devastating spells that can destroy the enemy infantry with ease, as well as summon more units or heal the ones already there. This turns their infantry into a slow but unstoppable tide of unbreakable, disposable corpses. They also have decent cavalry and monstrous infantry, and their monsters and chariots, despite not being particularly fast, can endure plenty of damage. This enables them to grind out victories that scarcely seem credible on paper.