Explain the Vector Addition of Velocity

Of course the velocity is relative to how you indicate the position d. If you are moving in water that is itself moving, then your full vg is the resultant of the sum of your motion v₁ in the water and the water's motion v₂. Your velocity as seen from the shore is v_g = v₁ + v₂.

Velocity can also be resolved into x and y components like any other vector. Sometimes it becomes convenient to resolve the velocity into radial and tangential components.  v = v_r + v_t  

Acceleration a is a vector which shows how fast velocity changes over a time interval.

A = Δv / Δt

Acceleration can also be resolved into x, y components, or radial and tangential components.