Yes,the volume is the same for any speakers configuration,with a 4x12 you will gain more headroom.
The difference is in the sound:it will be hard to overdrive 4 speakers with a lower wattage amp and it will play almost only clean.You may need a good fuzz/distorsion pedal in that way.
Anyway,I think valve amps play good when they're pushed at least.If you play in locals,clubs a 100 watts is too much for push it.
A 40/50 watts amp is good enough or an AC30 head with a 2x12 is a good solution,but they "must" be 2 Celestion Blue Alnico speakers for the joy of your Vox.(and yours)