Make sure that not only are the wheels lugs tight, but that they are torqued properly. If the lugs are torqued unevenly they can cause the rotor to warp, which will then cause a vibration. My experience, nine times out of ten, a vibration while braking is caused by a warped rotor. Even a very slight warp, inperceptable to the eye, can cause significant vibration. Get yourself (or borrow) a dial indicator and check the rotor trueness.