A Vibration when applying the brakes in most vehicles can be attributed to warped Rotors. Warped rotors can have several attributing Factors. For example, Over tightened lug nuts can cause rotors to be in a bind therefore leading to warping during one of the next heating and cooling cycles. However, this is not say that this is the leading factor. Rotors tend to warp in general simply because of the number of times they heat up and cool during a year. When you stop suddenly or brake hard the rotors heat up and then cool back down. This often lead to an imbalance or warping. Replacement is the cure. You can often identify this failure by being able to feel a vibration through the break pedal when stopping.