Dancing Skeleton
Zombie
In a perfect would I would keep crumble. But there would only be two conditions under which a unit would crumble.
1 - A unit would take a crumble test at the end of the phase in which the general is slain. At the start of the next VC turn, a wizard using the Lore of the Vampires may assume control of the army. If this wizard is subsequently slain, repeat this process.
2 - A unit would take a crumble test if, at the end of it's turn, it is not within 12" of a Necromancer or Vampire, or within 18" of the army general.
I wouldn't require the Vampire or Necromancer to be using the Lore of the Vampires as I assume that the power required to keep a unit tied to undeath is such an innate magical ability for the VC wizards that they can do it even when utilising spells from a different lore. Keeping the entire army bent to their will requires more specific necromantic ability and as such, the general (and his substitutes) would still need to be using the lore of the vampires.
Crumble (in it's current incarnation) doesn't fit the fluff for several reasons.
Have you ever seen a Zombie movie where our heroes slay one zombie, and another one nearby drops dead out of sympathy? No, it's ridiculous. Why would brainless undead care that they are losing a combat? Its the magical will of the general that keeps them alive and motivates them. If anything, when their numbers are reduced through combat he should be able to focus MORE energy on them, not less. And finally, why would a centuries old Vampire or a savage bloodthirsty monster like a Varghulf care if some zombies or skeletons are slain in combat? Can you imagine it? "Oh no, some of my numberless and utterly meaningless minions have been slain! The pain is too much, I must die with them!"
No.
I mean, it's easy and it's simple, but it's not the best way to represent the nature of the undead.
The problem with everything you just said is it that it doesn't consider game balance at all.
If your troops didn't crumble within 12" of a Vampire/necromancer or 18" of a general then there would basically be no crumbling at all. So we would have unbreakable troops that don't crumble and can be raised with IoN.
I can also buy it from a fluff perspective. You compare it to a zombie movie, but keep in mind the rules for zombies aren't always the same. It is after all a fictitious creature. In the world of Warhammer you have a horde of undead minions not brought back by a curse or a disease but by magic from one being. Not an easy task. Having to maintain control of that link would be difficult and when minions die that link is weakened and others go down as well. But that's not to say you can't raise back more with a bit of concentration.
I agree it's stupid that a Vampire can crumble from skeletons dying around it. It doesn't make sense, but from the perspective of game balance I can respect it. If they changed this somehow it would have to be balanced out with a negative rule. You can't just fix things for fluff and leave it alone.