The speedbrakes / spoilers issue crops up on here every so often and the search function might help you learn a bit more about their use.
They serve two basic functions in flight, either:
1) To increase the rate of descent whilst maintaining the same IAS
2) To reduce the IAS while maintaining a fixed rate of descent
We can extend the speedbrakes with flaps extended on the 757/767, up to Flap 20. The drag associated with Flap 25/30 settings combined with speedbrake deployment would be very high and the descent rate would therefore be excessive, and the vibration is an issue.
Below 800ft Boeing suggest no spoiler extension whatsoever (with associated warning light) but most airlines round this up to 1,000ft AGL (which is also the common "must be fully configured for landing" point).
On many aircraft there is a "flight detent" on the speedbrakes (a position beyond which the speedbrakes can only extend to when the aircraft is on the ground). On the 757 there are six spoiler panels on each wing, and panels 4 and 9 (inboard spoilers on the outer set) only extend on the ground. On the 767 all panels are used both on the ground and in flight. The speedbrakes extend to a greater maximum angle on the ground than in flight.
Similarly some of the spoilers operate with the ailerons to assist in roll control.
On the ground the speedbrakes are used to "dump" the lift generated by the wings and thereby placing almost all the weight of the aircraft onto the main gear, which reduces landing runway required by increasing friction and allowing the wheels to grip the runway surface better and therefore allowing a higher braking level to be used.
It is a common misconception that the spoilers actually produce aerodynamic drag to slow down the aircraft on the ground. Whilst they clearly will do this, the effect is very minimal in comparison to the primary function described above.
Similarly in flight the actual parasite drag they induce is smaller in comparison to the lift destruction, which is the primary purpose.
Hope this rather long explanation helped you understand a bit more about these things!
I used to love the smell of Jet-A in the morning...