Directional influence does not exist in Smash 4. It was a mechanic that let you influence the trajectory of your launch. For instance, traveling at a 90 degree angle would require less knockback to KO than a 75 degree angle. The hypotenuse is always a longer distance than the height.
Vectoring replaces DI in smash 4. Its implication has much more subtle results. Instead of influencing the direction, vectoring lets you influence the magnitude of the launch by holding a direction.
Now, the confusion occurs when you vector to certain angles, making it appear as if it is DI. If you are launched 90 degrees upward and vector to the right, you are not changing the trajectory, you are changing the flight. Imagine DI as changing the angle of a cannon, and vectoring as spraying wind on the cannonball as it flies, changing the direction. You can spray the air head on, decreasing the distance travelled, or spray it at an angle, changing the direction horizontally, which is where it is confused with DI.
It may not seem like much in competitive environments because you arent focused on the details, but it works. If you want tangible results, go to training and throw bobombs at yourself and try different vectoring angles. You'll find that holding the direction opposite of the angle you were launched will shorten your travel distance.
