I had THE SAME vibrations that you describe a while back. I went as far as installing new brake rotors/pads and still the vibration was there. Took it back to Stevens Creek BMW and they told me my rotors are warpped, literally 1 months after I installed the new ones ! So i decide to take it to Fremont BMW and they diagnose for me. Turns out is was my Lower right control arm (it was bent) that was causing the vibration when braking. $900 Later and a new control arm, and no more vibration.
You know me !