Question

why do we have to subtract that vector with respect to whom we have to find out the relative velocity of the other vector from the later vector??

Answer 1

Let us talk in terms of distance first for easier understanding.
Consider it this way, on a race track,
At time 10s into the race
Player 1 is at 500 meter mark and,
Player 2 is at 700 meter mark.
So if you want to the relative distance between them. we need to subtract 500 from 700.

Same is the case in velocities, one vector is has velocity f(x) and the other has g(x).
Finding the Relative velocity will require you to subtract one from the other depending on the Question.

Hope it helps