With due deference to those who know the subject, I think we need to be careful with our definitions here. The historical airworthiness requirement is to establish a speed to fly in rough air which will provide protection from stalling, on the one hand, and structural damage on the other. The definined gusts from which protection is sought are 66fps at
Vb, 50fps at V/Mmo and 25fps at Vd (these will doubtless have been tweaked and changed into metric with the onset of JAR). The (old) definition of
Vb, therefore, is the design speed for maximum gust intensity - ie the max speed at which an aircraft type can encounter a gust of 66fps and not break up.
Vb is, therefore, one of the parameters used to establish Vra.
As for when to reduce to turbulence speed, I think passenger comfort will normally dictate that moment before any concerns for structural integrity come to the fore, but I'm not a freight dog!
Perhaps it's time for JT or NW to weigh in on this one!
Cheers all,
mcdhu