I guess speed is a huge factor in this. Much like with DI, if you can predict where or how your opponent can vector, they still are in hitstun, and you may be able to follow up
Partially. You can literally add distance with the same amount of hitstun though. That means that, unlike DI, you have an option that is practically always the 'right' option rather than it being a mixup. This is because if you vector away from your opponent during a combo, they have to literally spend more frames to catch up to you because you received more knockback than expected with the same hitstun, giving you that many more frames to escape.
2 Questions:
1. So do I always want to hold directly against the knockback trajectory to survive kill moves?
2. Can we categorize this as just a different type of DI (after all, the direction you hold is still influencing your trajectory). Am I allowed to call this VI instead? lol.
edit:
three questions actually:
What about multihit moves like jab combos? Should I just hold towards the angle it's already sending me?
1. If you were sent perfectly horizontally, you would probably want to hold up and towards the stage. The x-component will be the same as if you held just towards the stage, allowing for survivability, but you also gain a bit of vertical knockback which can make recovering easier. Same general logic as DI, but adapted to a new mechanic.
2. If you are sent straight up vertically, you can reduce your knockback without changing your direction whatsoever. You are vectoring to survive, but you are literally not influencing your direction whatsoever. This is why DI is an incorrect term.
3. We're not entirely sure about moving mid-hitstun/freezeframes if that's what you mean. (SDI is presumed to depend on a similar mechanic, but it's still not 100%)
So umm, is this regarded as a bad or good thing by the competitive smash community?
It's sort of neutral. It's change, so we'll have to adjust regardless. Some people will prefer it, others will not. Just like every mechanical change in smash 4.