The Netherlands international was in Birmingham last week to discuss a potential move to Villa Park with manager Paul Lambert admitting it would be a ‘dream come true’ to join the club.
Vlaar is keen to test himself in the Premier League after eight-years plying his trade in the Eredivisie but the defender received no further contact from Villa.
De Kuip technical director Martin van Geel feels it is unfair of the Midlands club to keep the 27-year-old guessing over his future and that he should set a deadline for Lambert to decide.
“If I was Ron, I would set a deadline,” Van Geel told the Dutch media. “I understand that his agent will do that.
“Ron has to know where he stands, and so do we. This isn’t fair, especially towards Ron.
“As a club I think you are morally obliged to show clarity towards a proposed new player. Now, it’s only confusing, not just for us, but also for Ron.”
He told NUsport: “It is good when you invite a player and give him a positive feeling but it seems to me that then you need to try a direct transfer and complete it if you want the player.
“They have so much money but they do not pay for him.
“I need to gradually know where I stand. If it can’t be guaranteed that Ron continues here then in the final friendly matches I will play without him in the centre of the defence.”