Vlaar should be back this evening.
Lambert said: "His absence affects things because of the balance. We have two left footers playing at centre-half which is not ideal.
"The two boys do great every time I ask them to go and do it. If they make mistakes, they are honest mistakes.
"But Ron gives you the balance - that's the big thing.
"Ron being there gives you that experience too. He's been around the block so he knows what it entails.
"He has got better. It took him time to adjust to it. But this season, he's been very good."
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