This should not be about technicalities, because then we become a community who is too easily bothered by rhetoric and technicalities that we risk a lot of what many other competitive communities value - how attractive the scene is to join to outsiders.
It does not matter what DI meant in past games, the words in the name still effectively convey how it works in Smash 4. It's like 'mana' and how its mechanics are different in many games, yet the word 'mana' still conveys what its all about. DI has become a word like that.
There is zero confusion in any series about calling a mechanic something still even if it changed in the sequel/patch/whatever. There is no lack of clarity, nor an abundance of confusion. What is confusing is why a community who plays nintendo's fighting game competitively would be a group of people who are so absorbed in technicalities that they'd name a mechanic in their game after mathematics language. Not caring about that means we risk going further down the rabbit hole, until our scene is difficult for new players to get into.
Starcraft and LoL are more newcomer friendly when it comes to lingo and the 'personality' of their game mechanic names. Their names fit the game, feel logical, and not elitist. Smash shouldn't be the black sheep when it comes to this important element of competitive gaming. We can't not care about how difficult our game is to integrate into large tournaments.
We can't let Smash be the game people tune out for and close their stream window for, due to commentators tossing around words like Vectoring and other words that make no sense unless you read a forum post. Not being able to succinctly explain a mechanic, because it's so poorly named, is bad and a thing communities logically avoid. That's the whole point, and challenge, of picking good names for things.
You have to quickly explain mechanics to people who haven't seen high-level play of your game before often as a commentator at big events, because part of your job is making the game make sense to people outside of the community. If you can't do that, you're toast. Which is why commentators especially won't be tossing around words like VI and Vectoring - because DI makes their job easier. That's a clue to you all that it's a better word - it's easier for the professionals to explain. The only way you can explain VI easily is if you ignore the word 'vector' at all in the mechanic. Which means it should not have "vector" in the name at all.
Part of having your game at a large tournament and commentating is "SELLING" the game as a spectator experience - you have to explain basic mechanics, and it is very easy to alienate people with bad rhetoric during this stage. It matters, and sticking with names like "vectoring" gives the people on the ground - the commentators, the teachers, and the players - an uphill battle when it comes to making the game sound fun and engaging to new players and audiences.