Chelsea have seen a £22 million bid turned down by their London rivals Tottenham for the signature of Modric, who joined Spurs from Dinamo Zagreb in 2008.
The Croatia international confirmed in June his interest in signing at Stamford Bridge, but manager Harry Redknapp is reluctant to sell.
Van der Vaart, though, accepts Modric's ambition to lift silverware, telling Sky:"If someone wants to leave it's difficult to keep them. When a player is unhappy it's not a good thing.
"Luka and (Gareth) Bale don't play for money - they play to win something. If they have to go to another club it's their decision. But I hope they stay because they are fantastic and really important for the club."