Rothosen are reportedly in a dire financial state and may be forced to listen to offers for key players, such as highly-rated South Korean Son Heung-Min.
The 20-year-old has recently attracted interest from Manchester United, Chelsea, Arsenal and Tottenham Hotspur, but Van der Vaart believes that it is important the club keeps hold of its best players.
"It would be great if we could add to the squad," Sky Sports News quotes the 30-year-old as saying.
"We need more experienced players with a winning mentality, but I know there is not much money to strengthen the team.
"So it is important to make sure our best players stay."
The Dutch midfielder left the Bundesliga outfit for Real Madrid in 2008 but returned for a second spell with the club last summer.