Soldier Luck does not affect Vampire Lords; as for "Stun," that's a fact.
It seems to be a "BUG."
Added 1 minute ago
It's based on experience. It's like "Lord of the Dead" for the Necromancers, "Barbarian Luck" for the Orcs, and so on.
Added 1 minute ago
If we're talking about experience, then with Soldier Luck, Blind works more often for me, even if the attacked unit is stronger than the attacking Vampire unit. Except in cases where the attacked unit is >>>>> stronger than the Vampire unit.