He's only missed a run of 1/2 league games twice in two years. On top of that he's had two longer spells out. He also played 24 consecutive league games. It's not like he hasn't been able to string a run of appearances together.
Lovren, Mangala, Koscielny, Dawson, Agger, Nastasic and Terry for starters.*
No CB in the world's absence is as catastrophic as Vlaar's seemingly is to us so that's not a fair question.
*Edit - He's missed less games injured in his time with us than these players have in the same time period.