a twin and an AC30 are very different animals
on the CC Voxs (which i own) you need to turn the blended channels up and then set the master volume to whatever you want in terms of loudness. So for all intensive purposes treat the Top boost or the normal channel (or both if youve chosen that path) as a gain knob and the master volume as the ....master volume. its hard to really get into a vox properly at these shops so i empathize.
just remember with a lower wattage amp like a vox the louder it goes the more grit youll have.
conversely if you want a great clean tone, crank the master volume and turn down the channel volumes. good luck to you sir and gratz on a great amp!
if there is no master volume you simple need to just turn the amp up.
Voxs drive very early and they have a great gritty tone.
edit: you can do what was mentioned about ^ but you really wont need to. Twins are difficult amps to break up. Voxs are designed to break up very early or at least take a lot less effort to distort. so you CAN do it...but you wont need/want to.