Reason for vibration on 99 Honda accord while braking?

Normally this is caused by a warped brake rotor. Remove the front rotors and have them inspected and replaced or turned at an auto machine shop. Replace the brake pads even if they appear to be good. Do not put the old pads back on a new rotor or trued rotor. Make sure the wheel lug nuts are torqued to factory specifications as over-tightning the lug nuts will warp the rotor. Heavy braking which causes over-heating of the rotors will also warp them.