I believe
Vb is determined during certification to give the best margin between the likelihood of stalling and exceeding Vmo through degredation of control during turbulence.
In the 737 (3/4/5) I normally slow to a lower cruise Mach number in bumps, (providing their is sufficient margin in minimum IAS/Mach), because the ride dramatically improves at .7M and below. I think Checkboard has it right, with one big CAVEAT - don't forget that sometimes you will need to ACCELERATE to
Vb!