Tanks dont really have a "choice" as they will be forced by the Content and/or the Community to take a certain path just like the current situation.
Vit would be more relevant if it raises your pDef and mDef, then you would have a real choice between Def or Offense Tank.
And people dont buy Pentamelded fending gear because of the option to do so, its just they to go if you want to be viable at the current End Content and either go slaying or fending doesnt give you the benefits a pentamelded acc give.