Every unit in the
VC army book has the Undead special rule, i.e. they're all Unbreakable and Unstable etc. If the general dies, each unit has to take a
Ld test, and it loses one model for every point you fail by (most commonly known as "crumbling"). If you have another Wizard with the Lore of Vampires in the army he sort of takes on the mantle of General, and stops your army crumbling. If he dies, the army crumbles again. If at any point you have no Lore of Vampire casters left, your army crumbles every turn.
Every Wizard can choose to swap one of his spells for the Signature spell. You don't "automatically" get it, though you can choose to swap to it if you wish. So while you do have a 100% of being able to know the Signature spell, you may want to stick with the spell(s) you've rolled instead. Unlike other spells, more than one Wizard can know the same Signature spell. Some signature spells, like Invocation, have different power levels. Each power level still counts as the same spell. Before rolling to cast the spell, tell your opponent which power level you're aiming for, and your dice roll has to beat the casting value of that particular level. If you don't beat or equal it, the spell fails, you can't choose to then have it cast on a lower power level.
Hope this makes things clear