Olderair the answers vary from pilot to pilot. What would only have me only flicking on the seat-belt sign may have another selecting turbulence speed. I dont think theres any written difinitivley (attitude\altitude\IAS changes etc) with regards to when you
must or should fly at
Vb.
In my own experience (usualy CB penetration) I reduce or increase speed to
Vb if my eye-balls start (or expect to start) bouncin up and down out of sync with the instrument panel.
I have heard some types (ex: 757)
Vb gives a smoother ride to the bums down the back even in only moderate turbulence.