Answer:
When the vectors have the same direction
Step-by-step explanation:
When vectors have the same direction their magnitude can be simply added to find a resultant vector with the same direction as the first two vectors.
Having the same magnitude does not mean anything when it comes to adding vectors because they might have different directions not making it possible to just add their magnitude.
When vectors have opposite directions they can be subtracted from one another being that the resultant direction will be that of the vector with the highest magnitude.