It's more or less the fact npc's die, meaning less quests. It's sad when it happens. But that's what autosaves and normal saves are for I suppose. I wouldn't mind if it were a bit more like hired thugs. Rather than just bursting in guns blazing. But then I herd someone complain about things never starting beofre you arrive.
Actually, you're wrong. the vampires don't kill essential NpC's. Only non-essentials die, therefore, no quest givers are gone.
Most shop owners are not essential, and even if a couple of them die, usually another member of the populace picks up the slack and runs their shop/stall.
But I've been through many vampire attacks now, and the essential NPC's take a knee while the non-essentials die.
Not to be an ass, but that happens in a hostile invasion. There
are collateral losses.