Before you add the __getattr__ wrapper, I think you need to get vmin, vmax and value like this:
# Get current state.
parent = self.parent()
vmin, vmax = parent._dial.minimum(), parent._dial.maximum()
value = parent._dial.value()
But then once you add the getattr wrapper you can access it directly:
# Get current state.
parent = self.parent()
vmin, vmax = parent.minimum(), parent.maximum()
value = parent.value()
The code was changed before the wrapper was added, but this correction worked fine for me.