The club captain has been sidelined for a month with a niggling calf injury.
"The younger guys in the team have done alright when they've been called upon but Vlaar is an international who was on top form before his injury," Big Ron told the Birmingham Mail.
"The defence is a very important unit and it seems to be getting better at Villa.
"If you're conceding two goals a game for arguments sake, which certain teams in the Premier League do, you're going to struggle to win.
"You are very unlikely to score three goals so you have to keep it tight.
"Whilst everyone likes to see free-flowing football, you have to have people who can defend."
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