The 25-year-old Croatia playmaker last month stunned Spurs supporters by declaring he wants to leave - with Chelsea his preferred destination. The north London club have rejected a £22million bid for Modric, with chairman Daniel Levy saying the player is not for sale.
However, Holland international Van der Vaart said on Sky Sports News: "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."
He added: "But I hope they stay because they are fantastic and really important for the club."
Van der Vaart was speaking on the day Spurs players returned for pre-season training, seeking to make up for the disappointment of missing out on Champions League football in the forthcoming campaign.
The 28-year-old added: "Drop too many points and you don't deserve to be in the Champions League. So next season we are going to focus on the Europa League.
"As a player you want to improve every year but the important thing is to help the team. If we stay together we have a good squad."
One player who has arrived at White Hart Lane is Brad Friedel - the 40-year-old former Aston Villa and Blackburn goalkeeper having signed a two-year deal last month.
He joins Heurelho Gomes and Carlo Cudicini on Spurs' books, and Van der Vaart said: "I had a chat with him (Friedel) and he's a really nice guy.
"I think he's going to help us. We now have three fantastic goalkeepers."
Source: PA
Source: PA