I've concluded some testing of my own. I apologize if some of this is redundant, I'm just going to cover everything I have noticed.
First off, lets look at how we know DI was in the past. You hold a direction, and it alters your base knockback. Now with Vectoring, or as it should be called, Hitstun Shuffling, that's not necessarily the case. What do I mean? An example.
Little Mac is hit with a Bomb omb at 44% on the left edge of the right platform on Gerudo Valley. I hold no direction, and I wind up just before the left platform. I hold down now, and land where the bridge starts, under the sign post. I then tap down and I land in the exact same place.
That seems odd doesn't it? The tap and held inputs resulted in the same landing place. Well, that's because of how your momentum direction is determined. Upon any one hit (this is important later) you can input ONE direction to alter how you fly. What's important to note is that holding the input doesn't matter. Once the game registers the input, your flight path is set, and cannot be altered until you leave hitstun. So how does the game determine this?
Say youre holding right on a hit, the game would read that held input and then alter your flight path accordingly. Even if I began to mash left at this point, or hold down, or anything, my flight path would NOT change. In the earliest frames of hitlag is where you're input is determined. If you say, input nothing and then after you start moving a bit you input down, you're still considered moving with no directional influence since that's the input the game recorded.
Now for the actual Vectoring part of this. For simplicity sake lets say that Neutral is 0 and an Input all the way up has a value of 100. An example
I'm Dedede at 42%, on the lowest left platform on the right side of it on Gaur Plains. I input a vector value of 0 while in hitlag (again, with help of Mr Bomb Omb). I then proceed to get knocked back, flying just under the 2nd platform from the top on the right side. I then input a vector value of 100 in the same scenario, gliding a bit under the top right platform, and a bit above the 2nd top right one. Now, I do this one more time, holding the stick between 0 and 100 up (let's say at about 50). My flight path causes me to land on the left edge of the 2nd top right platform, where I can then tech. Meaning the "strength" of the input alters how you shuffle your momentum, and not just the direction. So where your stick is at the time of flight beginning determines how you move.
Finally, as mentioned, the ratio of Vectoring goes 1:1, 1 vector input for 1 individual cause of hitlag. If a move, lets say a Pit Fair, multihits (in this case, 3 times) then for each individual hit you can input a vector value for each corresponding hit, possibly allowing you to escape a multi hit move before the final launching hit. Holding an input does not work though....to a degree. A held input would alter the first hit and the last hits trajectory, but not any of the weaker multihits (or at least, the effect isn't as noticeable)
And that's all the research I've conducted over the last week.