I don't think Djokovic belongs in the same conversation as players like Vilas on clay.
And how many good quality victories does Djokovic actually have at RG during his career. A 30 year old Federer that played horrendously this year, Del Potro last year maybe and Gonzalez in 2006. It's not like he has particularly difficult draws there over the years.
I think that Djokovic has been a bigger beneficiary of surface homogenisation and hard court tennis being the norm on every surface (the make-up of the current tour is basically completely dominated by hard courters), than either Federer or Nadal.