Vibration may be the first sign of shimmy. Full-blown shimmy is really seriously nasty: basically the bike behaves like a gyroscope and cannot be steered. This may take you into the path of oncoming vehicles. I experienced shimmy at speed several times before I discovered the cause. Believe me, it was very un-funny, and I am grateful to have survived.
Shimmy can be caused by bent frame or forks, or a bulging tyre which unbalances the wheel. I suspect it may be triggered by applying the brakes, but they are probably not the cause. In my case, the cause was an out-of-true wheel. So please take my advice, and get your wheel trued. And if you are really confident that is already done, then worry like hell about the vibration until you track it down.