if the light velocity is a vector quantity, why vector addition cannot be applied to it?

Dear student,
Velocity of light like other velocities, is a vector quantity. So, in vector addition, veloicty of light is going to be added vectorially. Moreover, the velocity of light has a special significance because of relativity and therefore the way its added is under a relative sense.
If the answer isnt clear, please specify the context under which you want to say that vector addition cant be applied on it.
Regards