Guzan is not surprised at Vlaar's impact for two reasons - his quality and his leadership.
He told Villa's website: "Ron has been tremendous for us.
"He's an international defender who has played football at the highest level. His ability speaks for itself. He has commitment, strength and understanding of the game.
"What he's brought to the team has been massive. He's a true competitor - a real warrior, a real fighter. You need that in the Premier League.
"He's very vocal and a real organiser. That's important in any team. Ron brings that to the table."