Most Z/28's (and '64-'65 L76 and L84 Corvettes) I've worked on, with their original "30-30" cam, showed 9"-11" of vacuum at a 900 rpm idle.
Manifold vacuum has nothing to do with the operation of the ZL2 hood - it's operated electrically by a solenoid, from a plunger switch on the accelerator lever and a relay on the firewall.