I think perceived tiers largely reflect how easy an army is to use, and not necessarily whether or not a book is strong. You'll notice the "top tier" armies have two things (1) lots of units that can stand alone without support such as WoC core chariots, ironguts, beasts of nurgle, etc. and (2) units that can carry an army through sheer stats/abilities like the nurgle demon prince, greater demon and tooled-up slaan. Having not one, but both of these elements in an army make it much easier to play and significantly more forgiving if mistakes are made, therefore average to sub-par players can win games consistently with these armies. If an army wins consistently, it gets labeled "top tier". VC just don't have a unit that can sweep through an army on their own, and while an argument can be made that a level 4 blender lord is an uber character that can rub elbows with demon princes and Slaan, their effectiveness is somewhat offset by the fact that they only have 3 wounds and a 4++ save and more importantly, if they die, it's most likely game over.
Truth be told VC counters the current WoC power builds better than anyone else - they have the ranks and invocations to hold up monsters, ethereal units to hold up monsters and chariots and screams to kill a low model-count army that is mostly LD8 regardless of armor. On average 1 terror gheist scream will kill a 550pt Nurgle Demon Prince.