Its all hypothetical but the best CBs the premier league has ever seen, played in much more settled and established defences with more experience and debatably better players.
For example, lets take Terry. He played with Carvahlo, Ivanovic and Ashley Cole. I'd be hard pressed to find even a fellow Liverpool fan who would say Roberton is better than Ashley Cole, or Lovren matches the tactical application and intelligence of Carvahlo.
The same extends to Rio and Vidic. They played in absolutely phenomenal defences, infront of established premier league keepers who have been there for years. As good as Robertson has been, I'm under no illusions Evra is a clear tier above him in ability, leadership, command, presence, attacking ability and defensively at present. Playing with class acts such as these with multiple titles under the belts will make your life easier. That reminds me... the premier league's standards for defensive ability has dropped off a cliff the last few years. A prime Evra and Cole must be worth like 80M these days?
Taking this into account, I think the argument of VVD walking into a team with a terrible defensive record, with inexperienced and debatably poor players, under a coach who isn't best known for his defensive organisation or for setting up his teams to protect the defence... and fixing the defence is really really impressive. If VVD maintains this level of ability for the rest of his career, I struggle to really debate him being the best defender in individual ability that the premier league has seen.