rule question - vampire lord down to wizard lvl 0

BRB Official Update Version 1.9, p. 7:
Q: If a Wizard has had his level reduced to 0 does he still counts as a Wizard and can he still attempt to channel Power and Dispel dice? (p28)
A: Yes.

Note, however, can he cannot cast spells, even if he would still know them:

Ibidem:
Q: If a Wizard has had his level reduced to 0 and he still knows one or more spells, can he attempt to cast them? (p31)
A: No.


In any case, as you say, the AB specifies that the General must be "destroyed"/"removed as a casualty," before the army starts to crumble. So, even if he would not be a Wizard anymore, the army would not start to crumble, as long as he is alive - euh, I mean undead.

On a "general" note: the rules for determining the General are part of creating your Army list. Just like with other requirements regarding Army building, the game itself can and will make those requirements invalid.