As I understand it, VI influences trajectory,
only visually.
Behind the scenes, VI makes constant adjustments to the character's vector, as illustrated in the image below.
In this example, a character is sent upwards at a 90° angle, but left VI is used to influence the character's vector position to the left. It's important to note that the character never translates along the diagonal (black) slope, which is how DI would influence the knockback. Instead, in effect Smash 4 knockback is only applied along the Y-axis, while VI "teleports" you along the X-axis; momentum is maintained after you've "teleported."
As you can see, the vector influenced trajectory is the same (90°), and the total travel distance using VI is the same as the original trajectory travel distance. This is why vector influencing your character in a perpendicular direction to your original trajectory doesn't help you survive longer.