The Eredivisie side's technical director Martin van Geel believes it is unfair of Villa not to have made a decision on signing the 27-year-old.
"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."