how accurate is the little red line?
For you, on any given day? It's not. It reflects a demonstrated certification number obtained by the manufacturer. Any change in weight, center of gravity, technique, configuration, air density, propeller efficiency, power setting, aerodynamics, vertical gust (turbulence), etc, will have an impact on that figure.
Vmc as a certification number may be more or less than the speed at which you may find a control departure in your aircraft on any given day, depending on numerous factors.