Solve the actual problem and show that the vectors are different.

 

 

Still unanswered.

So the quote up top is incorrect?

Even if all it does is return to the original starting position by reflecting around the box?

That type of velocity measurement is the net or average velocity over an interval and is dependent of the interval over which it is measured. If the width of the box is L, you get a net velocity of 0 if you use intervals of 2L/c. If you use intervals of 3L/c, you can get a net velocity from anywhere from 0 to c, depending on where you start and end the interval.

 

This type of velocity measurement is very limited as it is too dependent on an arbitrary choice of interval.

 

It is the instantaneous velocity of light which can never equal 0, and when we talk about the velocity of light we are talking about the instantaneous value.