People have to learn to distinguish between legacy and raw performance/ability to have an honest conversation about this.
VVD does not have a legacy yet, only the past 2 years has he been at a relevant stage. But it is simply a fact that Van Dijk this year has had one of the greatest defensive seasons in EPL history, if not the greatest. Many would argue only 2005 Terry came close to this, and maybe the likes of Vidic somewhere behind both of them.
It is also a fact that Van Dijk is arguably the best (at worst 2nd best) player in what is one probably of the 5-10 best teams in EPL history. This Liverpool team would likely beat the piss out of most historical EPL champions. Van Dijk is the only defender other than John Terry to win a Player of the Year award in England.
I will say that not many of the defenders many consider among the greatest of the past 20-30 years have individually transformed a defense like Van Dijk has this year. A vast majority of them gained their historical status by being a top 3 CB or so over a long and decorated career, it's actually extremely rare that a CB is clearly the best in the world for several seasons in terms of actual performance. His peak is as high or higher than any CB of the 2000's era, but that alone isn't worth as much. Many of those CB's have at least 1 similar standout season and then many years of being among the best to go with it. Hell, based on 1 year alone, Umtiti at his peak for us wasn't far from this conversation. But that's only a year or so being considered.
However, it would be disingenuous to downplay the fact that Van Dijk is anything less than dominant. On top of that, it's his versatility and being dominant at so many different aspects of the game that makes him feel like such a presence on the pitch.