Perhaps this article in the
Boeing Aeromagazine No. 12 answers some of your questions.
For an Airbus FBW airplane like the A320, Vs1g is the lowest speed attainable in normal law at 1g with the sidestick held fully back. At low altitude it is defined by alphamax, an AoA slightly below that for aerodynamic stall. At high altitude (i.e. above a certain Mach number) Vs1g is equal to the 1g buffet onset speed.