Vlaar was El Ahmadi's captain last season as the duo helped Feyenoord finish runners-up in the Dutch top flight and book a Champions League spot.
"I was looking forward to linking up with Karim again," said Vlaar.
"We sat next to each other in the dressing room at Feyenoord. We know each other really well.
"To have Karim here makes it easier for me to settle and become a member of the group. I will feel at home.
"It certainly makes it easier. But even if that wasn't the case you find your way. We will see what happens.
"I have heard some good stories from Karim about the club, the way of living here in Birmingham - the area is fantastic - and the training."