Vso, I seem to recall, is the "minimum" speed in the stalling maneouvre (power off), which may be well below the speed at which the aircraft first starts to stall.
Has it been determined that if a manufacturer were to determine both 1.3 Vs0 and 1.23 Vsr0, would these two speeds now be about the same, for conventional aircraft types?
Perhaps MFS can provide some more insight.
Hawk