Sounds like your pushing the brake pedal hard. In the old days you would skid. Now the ABS system is automatically releasing the brakes and re-asserting them, hence the vibration. However if they vibrate constantly then you have issues that need to be taken to a garage.

What Year/make /model is your car? What type of surface were you on? The older your car, the less sophisticated the ABS. Early ABS (1990 - 1995) would reduce brake line pressure to all wheels if wheel slip was detected on any one wheel. Even with newer systems, skilled drivers can stop faster without ABS than with ABS. A modern ABS reacts to wheel slip in several ways. It shuts valves to close off the hydraulics that allow the driver’s brake pedal to increase brake line pressure. Provided the driver keeps full pressure on the brake, the ABS Solenoid rapidly toggles the brake line pressure on and off to keep the wheels turning. This tends to increase stopping distances but prevent skidding. If one is driving around a curve then skidding is not desirable because one would tend to skid off the road. However, if one is traveling in a straight line then a shorter stopping distance is always preferable. While it may seem counter intuitive, skidding is frequently preferable when emergency braking in a straight line. When drivers lock-up the brakes to the point of skidding, the vehicle travels straight, even if the driver turns the steering wheel. With ABS, the “un-skilled brake-pedal to the floor driver” retains the ability to steer. Unfortunately, this steering ability under hard braking contributes to a significantly higher rollover and run-off rode accident and fatality rate among drivers with ABS equipped vehicles. Under hard deceleration, a vehicle that turns suddenly tends to either flip or "sling shot" off the road. This is more likely if the driver is either unskilled or unsettled by the longer stopping distance afforded by ABS. Think of it like this. If a collision with a vehicle straight ahead seems inevitable, drivers may resort to panic steering. Panic steering with the wheels locked effects minimal change in direction. Panic steering with full steering control causes the car to whip into a new direction. The extent to which stopping distances are increased by ABS depend on the year of car and the surface. Stopping distances on Gravel, Ice and snow covered Ice are dramatically increased. According to http://wikicars.org/en/ABS A June 1999 National Highway Traffic Safety Administration (NHTSA) study found that ABS increased stopping distances on loose gravel by an average of 22% A Finnish car magazine, Tekniikan Maailma, Braking distance from 80-0 km/h with the following results: …………….........4-Wheel Lock-up (No ABS).....................................ABS Dry pavement.........................45 m.........................................................32 m Snow.......................................53 m.........................................................64 m Ice..........................................255 m........................................................404 m Keep in mind that these test compare ABS results to 4 Wheel lock-up. This is a fair comparison if we wish to measure the results for the unskilled driver. However, skilled drivers can perform much better without ABS if they use threshold braking, cadence braking or the appropriate amount of wheel lock for the road surface. On road surfaces like gravel and snow, a 4-wheel-lock-up provides the best results. Proponents of ABS (including the groups that lobby for ABS manufactures) like to attribute the shorter stopping distance to the small wedge of loose material that builds up in-front of a locked wheel. This makes the “locked wheel braking advantage” seem quite insignificant. No one would expect a few ounces of snow or gravel in front of a tire to do much to slow down a vehicle. While it is true that some material builds up in front of a locked wheel, the loose material is not the source of improved stopping power. Under emergency braking, an ABS equipped wheel is constantly rolling onto loose gravel or snow. Since the wheels are very likely to lock on this loose surface, a dramatic and prolonged reduction in brake line pressure ensues. This accounts for the dramatic increase in ABS stopping distances on Gravel, Snow, Ice and Snow covered ice. When a wheel is locked, it changes from a wheel into a “bull dozer”. The loose material gets pushed out of the way by the leading edge of the tire. This allows the contact patch to sit flush on the hard surface below for maximum stopping power. ABS equipped vehicles never get the opportunity to explore the traction that exists a few millimeters below a loose surface. ABS accident rate statistics are becoming more and more difficult to interpret now that virtually all new cars are manufactured with ABS. Back in 1991-1992, it was relatively easy to tell that ABS was increasing accident rates. The 1991 models without ABS had fewer accidents. All of these 1991 – 1992 drivers had the opportunity to become one of the skilled drivers who would typically out perform an ABS computer. Yet the 1992 drivers had more collisions. We are now raising a generation of drivers who have never driven a non-ABS equipped vehicle and have never had the opportunity to learn conversional braking skills. In away, driving school has been turned upside down. My generation was taught many different braking techniques for different surfaces. Modulating brake force and threshold braking were the key skills to learn. Standing full force on the brake pedal was not considered a skill but we knew it was the best way to emergency stop on a straight gravel road. We learned that pumping the brakes was a way to regain control if we had plenty of time to stop but that it was not an emergency braking procedure. Today, drivers are taught to stand on the brake pedal and let the car do the thinking for them. Most are not even taught to do this. In fact, most drivers know nothing about ABS until they experience what seems to be a brake malfunction during emergency braking. Panic brake pumping and panic steering are just a few of the likely outcomes. If you find your ABS braking experience unsettling, you may want to take yourself to a vacant parking lot with a few different surfaces and practice some emergency braking. While you are at it, you can also pull out the ABS relay and repeat the same tests. It may be eye opening for you to explore just how much traction remains beyond the threshold that you ABS computer deems appropriate.