Let define acceleration : the rate of change of velocity with respect to time t. Since velocity is vector quantity so will acceleration. Thus
If magnitude of `vec v` decreases then magnitude of `vec a` also decreases. But `vec a` denotes an acceleration, so we can say in this situation decreasing acceleration. At a moment when magnitude of `vecv-vecu` become zero object has no velocity.
Deceleration is slowing down. It is not a vector quantity. Some time it appears in a subject like Economics or biology. A decrease in heart rate below the limit when it is very difficult to recover normal heart rate, this is also known as deceleration of heart rate.
Decreasing acceleration is vector quantity while decelaration is scalar quantity.