Which player first popularised the use of Phantom over Vandal, according to you?
For me, it was clearly naTs during his Gambit days and also a bit of Buzz using phantom
The Phantom was popular from Beta, nobody needed to "popularize it", it appears you have an inaccurate image in your head where the Vandal was more popular in the beginning or something...
But, if you want to be pedantic, you could say brax "popularized Phantom", since he was one of the first big names to play Valorant Beta and had by far the biggest viewership during the early key drop period.
From early 2020 to late 2021 Phantom was actually more popular at pro level and in ranked (even though the numbers were always something like 54% vs 46% or around there). Starting with 2022 it balanced even more, with each of them, Phantom or Vandal, taking small leads in different periods of time but always averaging yearly around 50-50 share.
This is why I'm saying that your question doesn't really make sense, unless you're looking from a very specific point of view and timeline (eg: "I started playing during exactly this time, so I felt Phantom was less popular"). And if you're asking for the entirety of the game (which is my point of view), then the answer is: It was always popular, nobody needed to do anything about it.