Va deals with more than just popping wings. It deals with control surfaces of the aircraft. A full deflection could stall the tail before the wings. Of course Va is also being rethought by the FAA since there have been instances where full scale deflections still caused damage. Think of the airbus that sheered it's tail off after being caught in the wake of the 757.

Vb also deals with structural integrity of the aircraft I believe. IE those fancy CRJ900 or CRJ 1000s caught in some bumbs. The wing might never stall but that long body could snap or crinkle.... Since you aren't making full scale control movements in turb, where inertia plays a role, it's possible the control surfaces or wings won't stall before structural damage occurs while flying at Va. If I had to take a guess, which I would, I'd think it would apply to heavier aircraft or those with a smaller load factor per sq ft IE MD-80, CRJ-900s, 757etc.