Well, a variety of things can be responsible and alignment is one. It could also be balancing because that high speed would result in high centrifugal force.
But it all depends on a lot of things.
In your case you didn't indicate where you feel the vibration from (steering wheel or the whole body of the vehicle).
Since the vibration comes when you apply the brakes, the brake pads or lining could also be bad or worn.
However, if a mechanic gets a physical inspection of the suspension and the brake pads, he will be able to pin out what exactly is wrong.